Method for Inhibiting the Initiation and Propagation of Formation Fractures While Drilling and Casing a Well

1. Field of the Invention

This invention relates to drilling and casing of wells, and more particularly to a method of inhibiting the initiation and propagation of fractures in susceptible zones as the drilling progresses, and as the casing is installed in the wellbore.

In the drilling of a well into the earth by rotary drilling techniques, conventionally a drilling fluid is circulated from the surface of the earth down a drill string having a drill bit on the lower end thereof and through ports provided in the drill bit to the well bottom and thence back to the surface through the annulus formed about the drill string. Commonly, drilling fluids are employed that are either oil or water based. These fluids are treated to provide desired rheological properties which make the fluids particularly useful in the drilling of wells.

A problem often encountered in the drilling of a well is the loss of unacceptably large amounts of drilling fluid into subterranean formations penetrated by the well. This problem is often referred to generally as "lost circulation," and the formations into which the drilling fluid is lost are often referred to as "lost circulation zones" or "thief zones". Various causes may be responsible for the lost circulation encountered in the drilling of a well. For example, a formation penetrated by the well may exhibit unusually high permeability or may contain fractures or crevices therein. In addition, a formation may simply not be sufficiently competent to support the hydrostatic pressure applied by the drilling fluid and may break down under this hydrostatic pressure and allow the drilling fluid to flow thereinto.

It is this latter situation where the formation is broken down by the hydrostatic pressure of the drilling fluid to which the present invention is addressed. One of the limiting factors in drilling a particular portion of a well is the mud weight (density of the drilling fluid) that can be used. If too high a mud weight is used, fractures are created in lost-circulation zones with resulting loss of drilling fluid and other operating problems. On the other hand, if too low a mud weight is used, encroachment of formation fluids can occur, borehole collapse may occur due to insufficient hydrostatic support, and in extreme cases safety can be compromised due to the possibility of a well blowout. In many cases, wells are drilled through weak or lost-circulation-prone zones prior to reaching a potential producing zone, requiring use of a low mud weight and installation of sequential casing strings to protect weaker zones above a potential producing zone. If a higher weight mud could be used in drilling through weaker or depleted zones, then there is a potential for eliminating one or more casing strings in the well. Elimination of even one casing string from a well provides important savings in time, material and costs of drilling the well.

Thus, there is a need for a method of drilling wells using higher mud weight than could normally be used without encountering formation breakdown problems.

2. The Prior Art

Prior art techniques for dealing with formation breakdown have been remedial in nature, rather than preventive as in the case of the present invention.

Numerous techniques have been developed to control lost circulation. One common technique involves increasing the viscosity of the drilling fluid to increase its resistance to flow into the formation. Another technique involves the addition of bulk material, such as cottonseed hulls, sawdust, or ground walnut shells, to the drilling fluid. In addition, asphaltic materials including gilsonite have been added to both water base and oil base fluids to prevent the loss of drilling fluid from the well into surrounding formations. For example, in U.S. Pat. No. 2,773,670 to Miller, there is taught a method of correcting lost circulation by adding asphaltic material to drilling fluids. These materials are added to the drilling fluid in the form of discrete particles and are suspended as such in the fluid where they are carried down the drill pipe and thence via the annulus between the pipe and the well bore to the lost circulation zone. Upon reaching the lost circulation zone, due in part at least to the elevated temperatures and pressures found in the well, the asphalt particles attach themselves to the formation and are fused or bonded to one another and to the formation structure to form an effective seal against the passage of drilling fluid into the formation.

The use of gilsonite in suitable granular form in an aqueous carrier liquid for correcting lost circulation is taught in U.S. Pat. No. 2,812,161 to Mayhew. The granules of gilsonite are simply mixed with a portion of aqueous drilling mud and pumped into the well bore so that the mud will carry the gilsonite granules to the leaking strata and force them into the crevices therein. In accordance with Mayhew, the effective concentration of the granular gilsonite in the aqueous liquid carrier ranges from as little as 10 to as much as 50 pounds per barrel (ppb) of the mixture for most common cases of lost circulation, and for severe conditions this concentration may be increased to as much as 100 pounds per barrel.

In U.S. Pat. No. 2,573,690 to Cardwell et al. there is described a method of treating earth formations wherein a filler, which is a solid substance in granular form in two ranges of particle size, one being coarse and corresponding in size to the channel fissures and cracks, the other comparatively fine and corresponding in size to the particles of the formation to be consolidated, is used in conjunction with an earth consolidating resin-forming liquid mixture. Among those substances suggested as filler are walnut shells, pecan shells, coconut shells, and gilsonite.

Other granular materials used in the control of loss of fluid into formations include coal and coke. For example, in U.S. Pat. No. 2,650,195 to Cardwell et al., there is disclosed a method of controlling the loss of fluid into formations by providing a cohesive resinous coating upon filterable inert solid particles which are added to the fluid so as to form a filter cake of self-bonding particles when a thief zone is encountered. Various filler materials which are disclosed include coke, coal, cinders, and nut shells. The "coke" which has been used in these prior art techniques has been derived from coal by subjecting coking coal to partial oxidation in coke ovens.

U.S. Pat. No. 3,788,406 to Messenger describes addition of an emulsifying agent and a large amount of ground coal or asphaltic oil-wettable granular particles to control lost circulation.

U.S. Pat. No. 4,957,174 to Whitfill et al. describes a method of correcting lost circulation, after it has been encountered, by adding particulate petroleum coke particles in a specific size range to essentially plug off openings in a thief zone.

U.S. Pat. No. 3,053,764 to Hummel et al. describes a method of controlling lost circulation by addition to the drilling fluid of a mixture of large particles of coke and bentonite.

According to the present invention, fracture initiation and propagation during well drilling or during running in of a casing string is prevented or inhibited by including in the well fluid an effective concentration of a specially sized particulate solid material. The solid material inhibits initiation of fractures, and controls propagation of any fractures that are formed. The preferred inhibiting materials are nut shells or calcined petroleum coke, both of which are environmentally acceptable and do not significantly alter the rheology of drilling fluid even when present in amounts of 30 to 50 or more pounds per barrel of drilling fluid. Nut shells have a particular advantage in that they are very resistant to size degradation. Other particulate solid materials, such as calcium carbonate, glass or ceramics, polymer beads or the like can be used, but in some cases when using these other materials the drilling fluid rheology may be adversely affected, while in other cases handling problems, floating or settling of the particles in the drilling fluid due to too great a density difference between the particles and the fluid, size degradation or cost may be a deterrent.

The mechanics of operation of the method of this invention require that an effective amount of material in the 250 to 600 micron size range be used. Small amounts of material outside this critical size range may be present, but do not contribute significantly to inhibition of fracture initiation and propagation.

Inhibition of fracture initiation and propagation in accordance with the invention is effective with both oil-based and water-based drilling fluids. The method is not intended as a lost circulation control in cases where large fractures or faults are encountered, nor in situations where "thief" zones are encountered. The method is preventive rather than remedial in nature. The method is effective in preventing formation breakdown both during actual drilling and also when running casing into the hole or performing completion operations.

The method is very effective in pressure-depleted zones, which tend to induce lost circulation problems. The method is also effective in tectonically active zones, especially when drilling horizontally or at a high angle to vertical, such as greater than 40 degrees from vertical. The method is also effective in highly permeable shale or sand zones and in strong formations with highly directional in-situ stresses. The method is not appropriate for combatting fluid loss in vuggy structures, in zones having large open fractures, or in shallow zones that are loosely consolidated with gravel and/or large pores.

Prevention of lost circulation in accordance with the invention is far more cost-effective than remedial action taken after the occurrence of lost circulation. Especially when oil-based drilling fluids are being used, fluid losses can be very expensive, unsafe (subterranean blowout) and time-consuming. Thus, adding the fracture inhibitor to the drilling fluid provides a margin of fracture protection for the drilling mud weights used and reduces the degree of uncertainty involved with drilling operations. The procedure is particularly appropriate for use in lost-circulation-prone zones or in high-angle (greater than 40 degrees from vertical) or horizontal drilling where a higher mud weight is required to counter potential borehole collapse or compressive failure.

As mentioned above, the preferred materials for adding to the drilling fluid are crushed and sized nut shells or calcined petroleum coke. These materials are very inert, are environmentally acceptable, and have little if any effect on drilling fluid rheology even when present at levels of more than 50 pounds per barrel of drilling fluid. They are also within the preferred specific gravity range of from 1.2 to 2.0. Materials in this range are resistant to floating and settling in the drilling fluid. Other solid particulate materials may be used, such as calcium carbonate, gilsonite, glass and ceramics, polymer beads, etc. These materials may, however, affect the rheology of the drilling fluid to the extent that compensating chemical treatment may be necessary.

The concentration of material used may vary with the drilling fluid used and the conditions of use. The concentration must at least be great enough to provide an effective inhibition of fracture initiation and propagation, but should not be so high as to make circulation of the drilling fluid impractical.

In our earlier application Ser. No. 07/785,647, a preferred concentration of particulate material was defined by the following equation:

C: Concentration of particulate material in pounds per barrel of drilling fluid

SG: Specific gravity of particulate material

MW: Mud weight in pounds per gallon

A concentration that is within.+-.20% of the concentration calculated by the above equation was considered acceptable in the prior application.

We have now found that concentrations outside the range provided by the above calculation can also provide benefits. For example, higher concentrations, so long as they do not exceed the practical limits of pumpability, can be used with beneficial results. The use of such higher concentrations is more costly, and can lead to material handling and solid recovery problems, but nevertheless can be both useful and desirable in some instances.

The use of lower concentrations than calculated by the foregoing equation can also provide beneficial results in some cases, although the level of formation breakdown protection will obviously be less with the lower amounts. Nevertheless, considerations of cost and material handling may lead to a decision to use a lower concentration.

While a concentration significantly lower than the preferred level indicated by the equation C=SG(3.5MW-14.0) presents risk of a reduced fracture resistance enhancement, it has been determined from practical experience that "screenout" or "sandout" occurs during formation fracturing when the sand proppant concentration (sand SG=2.6) exceeds 32 pounds per barrel in the fracturing fluid. Since we want to achieve this "screenout" in our process in order to prevent fracture propagation and lost circulation, we have concluded that a lower concentration of solids than the preferred amount will also work, so long as the concentration is at least equal to the one obtained by the equation C.gtoreq.12.3 SG where C and SG are defined as above. This concentration is the level at which fracture bridging or screenout occurs during fracturing operations using solid proppant having a size range similar to the size range of the materials used in our process, and it is the bridging action that we are seeking, whereas in fracturing the bridging is an undesirable result. The advantages of using a minimum concentration are cost of material and minimum material handling problems, but the lack of safety margin when using a minimum concentration will normally lead to use of a concentration in the preferred range.

The critical size range of the fracture inhibiting additive has been determined to be between about 250 and 600 microns, or -30/+60 mesh (U.S. Sieve Series), with a preference for at least 75 percent of the additive to be in the -35/+50 mesh cut.

The mechanism by which the additive works is that as an incipient fracture is induced in a zone, the additive particles "screen-out" at the fracture tip due to fluid leak-off and resulting particle concentration at the fracture tip. The additive provides an excellent fluid sealing property and thus increases fracture resistance by "screen-out" continuously during fracture formation and propagation, preventing or minimizing loss of drilling fluid.

The effectiveness of the method has been demonstrated in both laboratory and field conditions as shown by the following Examples.

In this example, large blocks of Berea sandstone were subjected to pressure from a 16 pound per gallon (ppg) oil-based drilling fluid, first without the loss prevention material of this invention, and then with 80 pounds per barrel (ppb) of crushed and calcined petroleum coke (-30/+60 mesh) added to the drilling fluid. The fracture propagation pressure with the fluid containing added coke was from 1,700 to 3,300 psi greater than for the drilling fluid without the added coke. This result indicates that an increase in drilling fluid weight of from 3.2 to 6.3 ppg could be used at a depth of 10,000 feet without encountering formation breakdown or lost circulation. The freedom to use a significantly higher weight drilling fluid without losing drilling fluid into the formation translates into an ability to drill greater depth intervals without the necessity for setting intermediate casing strings. Elimination of even one casing string results in major savings in time and equipment costs. Also, it enables drilling of smaller diameter holes, with resultant economic and environmental benefits, especially in offshore operations.

A procedure similar to that of Example I was carried out using a 10 ppg water-based drilling fluid with and without 40 ppb of crushed (-30/+60 mesh) and calcined petroleum coke. An increase in fracture propagation pressure of more than 4,000 psi resulted from using the added coke in the drilling fluid. Comparable results were obtained using other drilling fluids at different fluid weights.

In this example, an oil field in Ventura, Calif. included a group of wells which could not be drilled with drilling fluid heavier than 11.5 ppg due to formation breakdown from the hydrostatic pressure of the drilling fluid. An openhole leak-off test of a well in this field was conducted using a fluid containing crushed and sized calcined petroleum coke. This test showed that a fluid weight of greater than 13.0 ppg could used without encountering formation breakdown in two depleted zones. The fracture resistance or breakdown pressure at the casing shoe was increased from an equivalent mud weight of 12.0 ppg without the coke to 15.8 ppg with the added coke.

In this example, formation breakdown tests were conducted in several formation intervals of a test well in Newkirk, Okla. In a permeable and competent sandstone formation, the borehole breakdown pressure was increased by an equivalent 8.0 ppg when 50 ppb of -30/+60 mesh coke was added to a water-based drilling fluid. The fracture propagation pressure in the same formation was increased by 5.0 to 6.0 ppg. During the test, observance of abrupt increases in the injection pressure shortly after fracture initiation indicated that coke "screen-out" had occurred at the fracture tip.

In this example, a well was drilled through an interval of 866 feet using a drilling fluid containing a 40-45 ppb concentration of crushed walnut shells (-30/+60 mesh) without any significant loss of drilling fluid. At the end of the drilling portion of the test, a static leak-off test was performed in a 4,366 foot interval of open hole. The fracture resistance at the casing shoe was increased from an equivalent mud weight of 12.2 ppg without the walnut hulls to 15.0 ppg with the walnut hulls.

In this example, a West Texas well was drilled through a depleted sandstone formation which normally presents severe lost circulation problems. The sandstone formation normally is drilled with a light (8.5 ppg) mud and then a casing string is run into the well and cemented to protect the sandstone when a higher weight mud, required for drilling into a higher pressure target formation, is used. In this case, solid material in accordance with the invention was added before the mud weight was increased, but the sandstone formation was not cased. No lost circulation was experienced from the use of the heavier mud even though the weak formation was not protected by casing. Avoiding the need for the intermediate casing string reduced the cost of the well. In this example, an additive concentration (40-45 ppb) greater than that provided by the equation C=SG(3.5MW-14.0) was used as a precaution, even though the higher concentration led to a greater cost and to some extent to greater material handling problems. Still, the savings from eliminating the intermediate casing string far outweighed these factors.

This example was a well having similar geologic conditions as in Example VI, but a slightly lower concentration of additive (35-45 ppb) was maintained, although still higher than the amount called for by the equation for optimum concentration. Also, in this case the additive was not added until the weak formation had been drilled through using a low weight mud. By waiting until the higher mud weight is needed, the total drilling time using the additive is minimized, and the handling problems are correspondingly reduced, as well as losses due to particulate attrition and cake formation on the borehole wall, providing even greater savings than when the procedure described in Example VI was used. Again in this case, no circulation loss was encountered and the cost of the intermediate casing string was saved.

The size range of the fluid loss prevention material in accordance with the invention is critical, and has been determined to be in the 250-600 micron range. Particles in this range are just slightly larger than the created fracture tip width and the thickness of the mud cake formed at the fracture surfaces. The plugging mechanism is a result of particle "screen-out" due to fluid leak-off and solids concentration at the fracture tip. Minor amounts of particles outside the critical size range can be tolerated, but the effectiveness is primarily due to presence of an effective amount of particles in the critical size range. Most preferably, a major portion of the particles are in the -35/+50 mesh range. Crushed nut shells and calcined shot petroleum coke are preferred materials due to their resistance to size degradation, inertness, density, cost and acceptability from the standpoint of environmental considerations. Also, they have little if any adverse effect on the rheology of drilling fluids even when present in high concentration.

The benefits of using the specially sized particulate solid material in accordance with this invention can also be obtained during the step of running a casing string into a well. Typically, in running a casing string into a well, a shoe is placed on the end of the casing to facilitate running the casing into the well.

In some cases, and particularly when the casing diameter is near the borehole diameter, lowering the casing string into the well creates a pressure surge due to the piston effect of the casing being lowered. This pressure surge can cause the pressure on the borehole wall to exceed the formation's ability to resist fracture initiation, and can lead to problems similar to those encountered during drilling with a drilling fluid which is too heavy for the formation.

It is possible in some cases that the formation can withstand the pressure from the column of drilling fluid, even without the added specially sized solid material of this invention, but that the pressure surge from running in the casing will exceed the formation's ability to resist fracture formation. In such cases, the potential for formation breakdown can be reduced by adding an effective amount of the specially sized solid material to the well fluid prior to running in the casing.

In some cases where the solid material was used during drilling of the well, there may be enough added resistance to avoid problems. In some cases, it may be desirable to add additional solid material to the well fluid to provide extra protection against the pressure surges generated during running in the casing, especially where the casing diameter is close to the wellbore diameter.