Completion of producing geothermal wells

Geothermics (i97o) - sv~CX^LISSVE2 U. N. Symposium on the Development and Utilization of Geothermal Resources, Pisa 197o. Vol.

_% Part

I

Completion of Producing Geothermal Wells U. CIGN1 *, A. GIOVANNONI *, E. LUSCHI * ^~a~ M. VIDALI *

ABSTRACT Completion of producing geothermal wells involves a series of operations. Following a brief description of spontaneous blowout, different systems to start production of wells are discussed. Should blowout fail to occur spontaneously from the well, several techniques (plunger piston, gas-lift and foaming operations) are applied. Mention is made of the decompression technique. When the output of a well does not warrant an economic exploitation, the geological and thermal data may suggest probing deeper. In this case, particular operational techniques are necess.ary to ascertain the possible productivity of deeper layers. A description is given of drilling and operations relate'~! to the round trip of the drilling string in presence of steam. Blowout

of a steam

liquid in the well is raised by conduction of heat from the rock and by intake at the pervious levels. The temperature increase and the emulsifying capacity of gas, although maintaining the static system in equilibrium, cause the hydrostatic level to rise as a consequence of the decrease in density of the fluid inside the well. In this phase, there are convection movements, with ascending and descending cycles, tending to equalize temperature in the entire mass of fluid.

well

Although the sudden release of a huge mass of steam and water from the depths of the earth, accompanied bv a sometimes deafening roar, can be very impressive (Figure 1), the blowout of a steam well presents no great danger for men and causes little damage to equipment. As a rnle, moreover, the blowout is controlled, except in cases when the hydrostatic head inside the well drops suddenly and falls below the pressure of the steam inside the layer. In such cases, which are discussed later, the blowout is sudden. In brief, there are two types of controlled blowout: a) Once the producing fracture is reached (during drilling of the vervious layer), and once mud circulation is lost, the hydrostatic level inside the well stabilizes, thus creating equilibrium between the weight of the head of water between the water surface and the fracture, and the pressure affecting the endogenous fluid intake area. As long as water is sent into the well (at its external ambient temperature), the rock constituting the well walls is cooled. Maximum cooling occurs at pervious levels having the greatest absorption characteristics. Under these conditions there is, in practice, no steam production, or at least not sufficient to originate a blowout. If the well is to start production it is left idle, that is, the introduction of water is stopped. Durin~ this phase, the ~aseous content of the endogenous fluid is released in the form of bubbles which emulsify the liquid column, while the temoerature ~enerated by the heat source tends to rise again, the cooling process having finished. The temperature of the * ENEL. Compartimento di Firenze, Gruppo zioni, Larderello, Italy.

Perfora-

FIo. 1. - - Blowout of a steam well.

When the temperature reaches the corresponding saturation pressure, the water vaporizes, with consequent considerable increase in volume, thus upsetting the equilibrium of the static system. Thus the fluid acquires a dynamic movement, generated by the fact that the capture area has a certain potential different from that of the wellhead area, where the potential is lower. As a consequence the column of heated and emulsified water is expelled violently from the well, starting the 757

blowout phase, which then continues with strong jets of steam and hot water, and the ejection of rock frag. ments. b) When the action of gas and temperature is not sufficient to originate blowout, due to the lower gas content of the steam or higher pressure of the hydrostatic head, blowout can be stimulated by decreasing the weight of the liquid column, for example by means of a plunger piston operation: this technique consists of continuous extraction of water from the well. Also for the purpose of lowering the hydrostatic head, various techniques involving the use of emulsifying or foaming agents or compressed air are sometimes used. One such technique involves the injection of liquid nitrogen, the volume of which increases considerably on conversion to gas, thus producing strong and violent emulsification, resulting in blowout. In certain cases, however, uncontrolled or spontaneous blowout occurs during drilling operations, when the producing fracture is reached, with the ensuing immediate loss of mud circulation and a marked fall in level. In such cases it can be assumed that the accumulation of compressed gas and the steam-water mixture overcomes the resistance of the head, causing rapid emulsifying and sudden blowout of the liquid column. The various methods of opening up steam wells are now described. SWABBING

The hydrostatic head existing inside the well at the end of drilling operations seldom decreases spontaneously sufficiently rapidly to cause a spontaneous blowout. Much more often, even when the well is idle, and therefore after a certain increase in temperature, the static equilibrium is not upset. This may be due to the excess of head above the crevice or to well dryness.

In either case the vlunger piston oneration is recommended in order to diminish the head, thereby stimulating blowout, or to check the reasons for lack of productivity (water seepage or complete dryness). The plunger piston operation, or swabbing, consists of lowering into the well 6 5/8" or 4 1/2" tubing depending on the size of the casing which lines the walls, to a depth of approximately 200 or 300 meters lower than the free water or mud table inside the well. The plunger (swab) is then lowered by sand reel cable inside the tubing to a level varying from 50 to 100 meters below the water table. The plunger, or swab, is essentially a cylinder with a ball valve at the upper end, opening upwards; externally it is fitted with rtibber seals of diameter slightly less than that of the inside of the tubing. The seals 758

are firmly supported underneath and capable of upward expansion. This unit is introduced into the tubing, together with a suitably sized drill collar, in order to increase the Overall weight and thus overcome friction against the tubing walls when the unit is lowered. The process is as follows: as soon as the plunger is lowered below the water table level, the liquid inside the well is free to pass above the plunger, its valve being open. The descent of the plunger is halted at a suitable depth, in order not to put too much strain on the cable and in order to avoid too abrupt diminuition of the differential pressure on the crevice. Then the hoisting phase is started, during which the valve is closed and the seals, pressing against the tubing walls, enable extraction of the liquid so collected. Most of the water remaining above the swab is therefore brought to the surface, and the quantity and temperature of the water are measured so as to ascertain, after a certain period of operation, the total amount of water extracted and therefore the progressive variation of level and temperature of water in the well. It may sometimes happen that, having lowered the swab too far into the tubing and into the liquid, it is difficult to raise the swab; in this event, gradual low-speed extraction is desirable. The following data are exemplary of a swabbing operation: a) assumed producing fracture at 800 m depth (initial leveh 400 m) b) 4 I/2" ~ 750 m

tubing lowered into the well to

c) tubing capacity: 7.8 liters/m d) upward swab speed: 6 m/sec e) calculation of water output: 7.8 liters/m × 6 m/sec -- 46.8 liters per second f) casing: 9 5/8" API, with a capacity of approximately 40 liters/ m. Thus, in theory, after 5 swab strokes, about 4000 liters of water have been brought to the surface, and therefore the level inside the well has decreased by 100 m. Number of swab strokes

Extracted Extracted water Extracted Swab water output water fishing tempera- R e m a r k s time in liters level at m ture

I

I0#

470

450

90 °C

2 3 4 5

I0" 15" 20 # 30"

470 700 940 1,400

460 500 570 570

90 °C 90 °C 95 °C 95 °C

3,980

Initial level at 400 m Final level at 500

m

During some swab strokes, the water output can be higher than the theoretical value, due to the help received by the mechanical action of the plunger from the steam beginning to move as a consequence of the decrease in the hydrostatic head. Other times it is noted that, notwithstanding continuous water extraction. the level does not decrease, or decreases not in proportion with the amount of water extracted. This event, usually occurring at the beginning of the operation, clearly means water is still entering the well; the problem is therefore to increase and accelerate the operation so that output capacity at the surface will be higlaer than the inlet capacity. In certain eases, even under these conditions, the swabbing is successful particularly when the well is a producing one and there is help from the steam thrust. In other eases, on the contrary, there is no practical possibility of coping with the water inlet, and the swabbing is useless. LIOUID NITROGEN

Liquid nitrogen is also used to empty the well quickly and therefore depress the assumed producing area suddenly, in the hooe of causing a blowout. Nitrogen, available in its liquid state at a temperature of --196°C, when injected into the well where much higher temperatures exist, increases ~'eatly and suddenly in volume and forcibly expels the liquid from the well. This procedure was used in exploration of a well, as outlined here. This well, run with a 7 5/8" easing down to 2000 meters, and with 5 1/2" slotted liners from 1948 to 2532 m, was placed under observation with a ground water table of approximately 120 m. After lowerin~ the 1 1/4" pipes to 1900 m for liauid nitrogen injection, pumpin~ was started with a special nitrogen pump having a delivery pressure of 700 atmospheres. The tank of liquid nitrogen contained 7340 liters. During inieetion the pump pressure ranged between 168 and 210 atmospheres. Fifteen minutes later water began to come out after inieetion of 640 liters of liquid nitrogen. All told, 1280 liters of liquid nitrogen were injected. After 30 minutes, injection of liquid nitrogen was discontinued, but the outflow of water and steam continued to increase. After 60 minutes gaseous nitrogen emerged, followed by steam and water both in considerable quantities. From this short summary of the operations, the importance of liquid nitrogen is evident, in emptyin~ steam wells which cannot be completed because of excessive hydrostatic head. This is a special technique calling for use of special equipment and resort to specialized service companies. COMPRESSED AIR Compressed air is used in the steam exploration fields of Larderello, again with the aim of reducing. the hydro.static head on the producing layers and eaus-

ing blowout. This theoretically simple operation is performed with a series of compressors, also used for compressed air drilling. Besides the compressors and corresponding pipelines for air delivery, of course, it is necessary to install either an airtight preventer on the wellhead or a rotating packer. Normal drill pipes are lowered into the well, or else casings with a diameter to be selected according to the drilling diameter and the delivery of the compressors. The pipes or casings are lowered to below the hydrostatic level, to a depth which again depends on the operating pressure of the compressors. Compressed air is pumped through the annulus between the casing and the drill pipes, or the regular casing and a casing lowered into the well in place of the drill pipe. Water and air are forced out from the inside of the pipes or casings lowered into the well. The effectiveness of this method depends of course on the capacity of the compressors available. If the necessary equipment is not available, the method must be evaluated economically in comparison with others that may be effective. Compressed air, though giving results similar to those obtained with mechanical swabbing, has a more consistent and effective emptying action. Moreover, it is important that the use of compressed air involves much less risk and danger than the mechanical swabbing. FOAMING AGENTS

When conditions allow, foaming substances may be useful for well completion. These agents help blowout by emulsifying the water column in the well. Obviously, the success of the operation is conditioned bv the level inside the well and the more or less marked presence of gas in the water. In the Larderello area, Howco Suds, a water-soluble surface-active agent was used. This product is very effective in lowering surface tension and produces considerable volumes of foam under different conditions. It is also used as a foaming agent when drillin~ with compressed air in the presence of water, and to restore production of water-choked gas wells. The use of foaming agents is restricted to those special cases when one wants to speed up the blowout, The assumption, on the basis of all available information, is that the blowout would take place in time without resorting to more radical operations such as mechanical swabbing, or emptying the well by compressed air or liquid nitrogen. Other products or mixtures of similar products were also employed. They yielded positive and even brilliant results but always in special cases. DECOMPRES S ION

When the presence of gas is observed in a drilling, when water temperature in the well is close to 759

the boiling point, and when the water level fluctuates, decompression is used to help blowout. Like foaming agents, decompression can be adopted with positive results only under certain conditions. The first is that it must be possible to place the well under pressure by closing the head valve, in other words, there must be enough gas to establish a considerable pressure,

I FIG. 2. -- Run-out operations in presence of steam.

Indeed, this extremely simple method consists in closing the well and then opening it suddenly in order to cause a sudden decompression on the water table in the well. The pressure attained just before opening depends on technical drilling conditions, gas-producing phenomena, and the pressure at the producing layer. To summarize, we can say that the choice of the most suitable method for completion of a steam well is made after a careful examination of the individual 760

conditions and data collected during its drilling. The most appropriate solution depends on both economic and technical considerations. R u n - o u t in presence of s t e a m

A problem closely connected with the completion of a well is the run-out of the drilling string or tubing. Cases when run-out must be performed are not rare. They either follow a spontaneous and sudden blowout. with the rock bit at the bottom, or blowout induced by swabbing. Or run-out may be considered advisable during the deepening of a producing well, as will be described later. This operation presents difficulties and risks as well as occasional highly dramatic aspects (Figures 2 and 3). When a well begins to blow out with the drilling string at the bottom, enough time must be allowed for the paroxysmal phase to come to an end before run. ning-out to recover the pipes. This phase is distinguish. ed by the wild outburst of mud, cuttings, and w a t e r The main difficulties are the poor visibility., tlJe deafening noise, the danger of bums from the outeoming fluid. and the possibility, of a relapse with new blowout of cuttings and rock fragments. Running-out in these conditions calls for a courageous, well trained staff, phys'.tally fit and with quick reflexes. Each individual movement mr, st be pre-arranged and individually coordinated to the group work. since the men can communicate little or not at all. All necessary safety precautions are taken, varying from case to case. It may even happen that, once the run-out is started, the thrust of the fluid is such as to lift the drilling string and in extreme cases to expel it. This is why it is necessary, before be~nning, to •re-arrange on the yard a system for anchoring the drilling string. and to fit on the mast a metal box to protect the derrick man. An emergency lifeline must always be kept in perfect working condition. Obviously. these operations require preventers fitted to the wellhead. It can sometimes occur, when the paroxysmal phase is exceptionally long, that because of erosion by cuttings the normally-placed wellhead goes out of service. In this case, the run-out operation becomes even more strenuous both for the staff and the materials and there is constant danger. D e e p e n i n g of producing wells

When the output of a well is not economically satisfactory, or when it is advisable to deepen it for better geological and thermal conditions, special operational techniques are adopted. We will discuss three of these techniques: water drilling with lost circulation, drilling with compressed air and endogenous fluid, and drilling with water and endogenous fluid. Water drilling with lost circulation

L

FIG. 3. - -

Run-out operations in presence of steam.

is the classic and specific method customarily used in steam wells whenever the circulation is lost at levels that correspond to the producing strata. Drilling with compressed air and endogenous fluid w a s first tried at Larderello and gave good results. Drilling with water and endogenous fluid was tried first in the Amiata area and proved superior for certain specific conditions. When drilling with air or water in combination., with endogenous fluid was not possible because of special conditions of the well, we resorted to choking the well with water and followed this with water drilling with lost circulation. WATER

DRILLING WITH

LOST CIRCULATION

Water drilling with lost circulation in drillings for endogenous fluid is the operation which asks for the greatest attention and special measures, since it is the most difficult to perform. Once the pervious and creviced producing layers are reached, drilling as ~i nile takes place without return circulation, since the water pumped into the well is absorbed by the crevices of the soil through which the endogen.ous fluid comes out. Water must be pumped into the well to lift the cuttings made by the rock bit to the nearest crevice, where they are absorbed with the water.

It is during water drilling with lost circulation that the main drilling accidents occur, such as rock bit stuck at the bottom and pipe breaking. For instance. if water circulation fails (possibly for drill-pipe breakage), the cuttings held in suspension fall to the bottom and cause sticking of the rock bit. The high temperatures, often reaching or exceeding 200 °C, further these stickin~s, since they tend to cement the fallen cuttings. In order to minimize these accidents, drilling must take place with little bottom deposit and minimum amounts of cuttings in circulation. The ideal is to remove and set aside all the cuttings produced by the rock bit. For optimum safety, the driller must frequently check the amount of deposit as he works, to ascertain whether the cuttings were completely disposed of. In water drilling, the penetration rate depends to a large measure on the possibility of cleaning and removing the deposit, as well as on the nature of the rock being drilled. A light rock, easily reduced to a very thin powder, is ideal for lifting by water circulation: a heavy layer is obviously more difficult. Also the choice of the rock bit is important. It should not cut the rock into large fragments; a bit with a short tooth is needed. and not much weight should be placed on it. The availability of water in considerable quantity helps drilling operation during this stage. The location 761

of the well or seasonal shortages may rule this out. If there is not sufficient water for a good operation (30 to 60 m3/h), it is necessary either to reduce the drilling hours or to work the cuttings into thin dust, so that they can be lifted more easily with a moderate amount of water. There are no fixed rules to be followed, since the problems to be faced are never the same. Only experience can suggest the most suitable solutions. The crevices encountered may be completely open or only partially open. Crevices completely absorbing all water pumped into the well are certainly to be preferred in view of the safety of the work, because the cuttings from the rock bit must be lifted only for a short distance before they are absorbed by the crevice. In partially open crevices, on the contrary, a considerable water column is present, carrying fine ground cuttings. A portion of these cuttings can reach the surface unabsorbed. As a conseqtmnce, any possible failure in the circulation system is dangerous. Another remarkable feature in lost circulation drilling is that since the well is often almost empty, the drill pipes are subject to a greater wear due to oxidation, and to heavier mechanical stress without the support of a good circulating mud. It is therefore important not to overwork the drilling string, to avoid too fast a rotation in order not to damage the casing also, to place little weight on the rock bit, to fit the pipes with protectors, and to check the existing deposit frequently and adjust penetration speed accordingly. Notwithstanding all precautions, drilling accidents occur, and in such events there is no other way but to resort to tool-fishing operations, which are often long and painstaking. It is very important, during water drilling, to pump a certain amount of mud from time to time as this helps lift the cuttings and clean the well. The mud is completely lost together with the water; nevertheless it is necessary, sometimes essential, for continuation of the work. A consequence of water drilling is a more fre0uent resort to cores, as only these enable us to have an idea of the rock being drilled. DRILLING

WITH

COMPRESSED AIR AND ENDOGENOUS

FLUID

This method was first tried at Larderello when. during compressed air drilling of an exploratory well beyond a depth of 400 meters, steam gradually increased and reached a maximum delivery of 6 tons/h, with no trouble ensuing and with a good drilling rate, comparable to the rate attained with air alone. As steam output from the well increased, an increase in air inlet pressure was recorded. In order not to exceed the compressors' nominal pressure, delivery had to be reduced by diminishing the revolutions of the motors. Drilling went on normally, in spite of the reduced delivery, since the steam effectively replaced 762

the reduced air volume in the circulation system to bring cuttings to the surface. Even in other drilling jobs, delivering much higher quantities of fluid, this method was adopted and positive results were consistently obtained, both from the viewpoint of operation procedure and from that of finding new producing crevices during the deepening stage. There are dangers in the working stage from the high temperature of the fluid, which damages materials, especially rubber parts, and from the jets of steam if the men are accidentally hit by them. Moreover the possibilities of the operation are limited by the quantity of fluid delivered and the avail: able equipment. Under these conditions, the theoretical stating of the problem is of little help. The endogenous fluid delivered by the well changes constantly in delivery, pressure and temperature. It is up to the operator to act accordingly at the right moment, sometimes by trial and error adjusting air inlet in such a way as to obtain optimum performance. DRXLLrNG WITH WATERAND ENDOGENOUS FLUID This kind of drilling was used successfully for the first time in the Amiata area in order to overcome some difficulties connected with the deepening of a few producing wells. In the exploratory, drilling, after having crossed several tens of meters of Rhaetic anhvdrites, the drilling started a spontaneous blowout, with output consisting mainly of carbon dioxide (approximately 90%) and with limited delivery due to small rock crevices.

The poor delivery was due both to the characteristics of the fluid encountered and to the crevice svstern of the rock involved. The problem therefore arose of deepening these wells with the endogenous fluid. since it was evidently impossible to continue drilling in any other way. At first the problem was solved with the use of compressed air, which made it r~ossible to deepen one well by 79.85 m (that is, from 471.45 m to 551.30 m~ in 52 hours of drilling, with an average hourly drilling. rate of 1.55 m and with an increase in fluid output of about 130,000 kg/h. Technical difficulties arose, however, from both the features of the compressors used and from the compressed air drilling method. It is known also that the presence of water, as was the case here, is a great hindrance for normal penetratiofl in air drilling. Later, when other wells blew out, all of them with limited initial deliveries, the same problem of deepening presented itself. In fact, these wells were in many ways similar to the preceding one which had been completed by air, but in these latter wells a new driHin~ technique was successfully tried out for the first time. This made use of minimal equipment required: a com-

bination rotating blowout preventer and stripper is ~imply added to the wellhead of the regular-driiling system. The drilling technique is just as simple, the aim of this being to deepen the well in the shortest possible time, and using available equipment. First we considered drilling with water and lost circulation, on the principle that the cuttings produced by the rock bit are carried into the open crevices by the water pumped into the well. However, we found that since the crevices are producing, the cuttings are carried from the crevice level to the surface by the fluid itself, together with the water pumped into the well. From the bottom of the well to the crevice level the cuttings are carried by the water alone. It is therefore a mixed method of drilling with water and fluid, which needs to be tried out each time in order to find the optimum fluid-water mixture to bring the cuttings to the surface. Fundamentally, this new technique could be also considered as a development of the compressed air drilling method, Its advantage is that it overcomes the critical point of the proper amount of water in the cuttings mixture. By adding water in such a way as to overcome this critical point, we create conditions like those of a drilling job with aerated fluid. On the other hand, we know that the carrying capacity of a drilling fluid is in direct proportion to its specific gravity; hence the greater specific gravity of the mixture means more

carrying capacity and as a consequence an increase in penetration rate. The results of two drillings confirm these observations. 1st deepening - Drilling from 751.10 m to 872.70 m, equal to 121.60 m drilled during 76 hours with an average hourly rate of 1.60 m. 2nd deepening - Drilling from 643.70 m to 730.50 m, equal to 86.80 m drilled in 25½ hours, with an average hourly rate of 3.40 m. To evaluate this new method, let us compare the3e rates to the average hourly rate obtained in the drilling completed with air (in similar conditions and soils). This was 1.53 m. It must be noted that. in all these deepening cases, a notable increase in fluid delivery was obtained. These few data suffice, in our opinion, to give a positive judgement on this method. As a conclusion, we may say that this drilling method could be adopted conveniently whenever conditions allow, in view of the minimum expense for equipment and operation. One may wonder if this new technique is appropriate in cases of a < hole where no more productive crevices are found. Our experience has been that the operation was satisfactory even in these cases. from the point of view of deepening

765