Summary Downhole mud/shale interaction can only be properly understood if rock mechanical, shale hydration, and fluid transport phenomena are taken into account. This paper presents a review of Koninklijke Shell E&P Laboratorium's research on borehole stability in shales. Mechanisms relevant to shale stability, including pore pressure penetration (the gradual increase in pore pressure resulting from high mud weight), capillary threshold pressures, compressive and tensile failure, post failure stabilization, hydration stress, inhibition, and osmotic phenomena are discussed. We attempt to integrate these mechanisms into a comprehensive model for shale behavior. Introduction Borehole instability in shales is a major source of trouble, time, and cost during drilling. Problems generally build up over time, beginning with fragmentation at the borehole wall, followed by transfer of fragments to the annulus, and culminating in such problems (if hole cleaning is insufficient) as a sticky or tight hole, packing off, hole fill, and stuck pipe. Consequences may include losing the hole, having to sidetrack, inability to log, and poor cementations because of excessive washouts. New technologies (e.g., horizontal, slim-hole, and coiled-tubing drilling) will not resolve borehole instability problems; they will lead to at least as many problems as conventional drilling. This paper addresses the mechanisms behind shale failure and the transfer of failed material to the annulus. A proper understanding and prediction of hole cleaning is equally important but requires separate treatment.1,2 Causes of Borehole Instability in Shale Borehole instability in shale is a complex phenomenon. Five basic problem areas can be distinguished:drilling through a naturally fractured shale,drilling through a brittle shale and inducing fragmentation through drillstring vibrations,causing shale failure with too high a mud weight (tensile fracturing),causing shale failure in a compressive mode through insufficient mud-weight support, andcausing shale failure in a tensile mode through hydration stress. In Case 1, the harm is already done, and only postfailure stabilization and optimum hole cleaning can help relieve problems. In Case 2, reduction of drillstring vibrations may offer additional relief.3,4 The third case, involving classic tensile fracturing, is not considered. This paper addresses the mechanisms underlying compressive and tensile failure and postfailure stabilization of shales. First, the effect of low shale permeability on borehole stability is discussed, followed by presentation of a poro elastoplastic model for rock mechanical behavior. How postfailure stabilization may alleviate shale problems is discussed, and finally, mud/shale interaction in terms of the shale's intrinsic hydration stress is addressed. Shales, like other materials, only fail if the effective stress state exceeds the failure envelope. This is valid for compressive and tensile failure at the borehole wall and also for failure of cuttings and cavings (disintegration, dispersion). Therefore, the models presented here are formulated in terms of downhole pressure, stress, and strength effects. Shales are relatively ill-defined rocks and may include both highly cemented shaly siltstone and weak gumbo-type shales consisting primarily of hydratable clays. Major differences in shale behavior can be attributed to these differences in composition. We define shale as a low-permeability rock where the matrix consists, at least partially, of clays. How Low Permeability Affects Shale Behavior Hydraulic Flow Through Shale. Shales have permeabilities ranging from ˜1×10–6 to 1×10–12 darcy. Because of these low permeabilities, no "normal" fluid loss occurs and no filter cake builds up on the borehole wall. Instead, gradual equilibration between mud and pore pressures takes place unless a barrier is present at the borehole wall. In the case of microfractures, a shale behaves as a dual-permeability medium, with high permeabilities in the microfractures and low permeabilities in the bulk of the material. With mud in overbalance, equilibration takes place from the wellbore to a semi-infinite medium and results in transient pore pressures, penetrating from the wellbore outward. Note that only a minor amount of filtrate invasion is required to raise the pore pressure over a considerable trajectory away from the wellbore. When drilling in overbalance, pore pressure penetration invariably leads to a less-stable situation at the borehole wall. To measure the rate of mud-filtrate invasion as a function of filtrate and shale composition, we have developed the microfiltration cell (Fig. 1). The principle of the test is simple: a confined shale core sample is put into contact with a simulated pore fluid on one side and with mud on the other side. Overbalance is applied to the mud, and the rate of pressure increase at the pore-fluid side is measured. The method has been used to screen muds and mud additives for their capacity to reduce or to prevent pore pressure penetration. The pressures applied were 3.5 and 0.35 MPa on the mud and pore-fluid sides, respectively. Mud is preceded by distilled water as a baseline measurement. After a set period (from 1 to 7 days, depending on the test), the pressure is bled off, the water is replaced by mud, the fluids are repressurized, and the test continued. Tests carried out so far on Pierre shale cores have already yielded interesting results. Fig. 2 shows the results of a test with a 3% KCl/bentonite/partially hydrolyzed polyacrylamide (PHPA) mud. The rate of pore pressure penetration is very similar to that of water. Micron-sized (and larger) particles and high-molecular-weight polymers apparently cannot plug off the shale surface, let alone invade the pore system. This can easily be understood because of the ˜1- to 10-nm pore size and the general rule of plugging by particles - one-third to one-seventh of the pore diameter. Fig. 3 shows the results of a test with sodium silicate solution (water glass). In this case the "mud" can reduce the rate of pressure penetration to almost zero. The silicate reacts with the divalent ions present in the shale pore fluid to yield a gelatinous precipitate that plugs off the pore system. Fig. 4 shows the results of a test with a 25% sucrose solution. Again, the rate of pressure penetration is significantly reduced. Apparently, the sugar molecules are small enough to enter the pore system and impart high viscosity to the invading filtrate. A similar advantageous effect of sugar was seen earlier in cutting-disintegration tests.5 Fig. 5 shows the results of a test with oil-based mud (OBM). Clearly, the OBM does not penetrate the shale. An explanation is given in the next paragraph.

Summary Friction and wear between casing and tool joint have been measured on full-scale equipment. The results show that the most important mud property, with respect to lubricity and protection against wear, is it ability to form a film that separates the two steel surfaces. Results fro small-scale tests are shown to be nonrepresentative. Introduction During drilling of deviated wells, the torque and drag of the drillstring and wear of the casing may become very high. This can result in considerable operational problems and increased drilling costs. The torque of a drillstring is generally determined by three phenomena: the friction between the drillstring and the casing (cased hole), the friction between the drillstring and the borehole wall (open hole), and the bit. Drag is determined by the first two phenomena only. Casing wear, of course, will occur only phenomena only. Casing wear, of course, will occur only in cased holes. In this paper, we discuss only those processes that take place in cased holes-i.e., the friction between the place in cased holes-i.e., the friction between the drill-string and the casing and the wear of the casing. The effect of mud composition on these two processes has been examined. Wear and friction are the result of a complex tribological process that occurs in the contact area between the tool joint and the casing with mud as the intermediate medium. Parameters-e.g., contact load, surface roughness, hardness, geometry, and chemical composition of both the tool joint and the casing-and mud composition will determine what kind of wear mechanism occurs. The wear mechanisms I addressed here are those relevant to tool-joint/casing contact:adhesive wear is the transfer of material from one surface to another during relative motion as a result of solid-phase welding;two-body abrasion is the removal of casing material caused by hard tool-joint protuberances; andhree-body abrasion is the removal of casing or tool-joint material by particles present in the contact area. The wear process determines the condition of the casing and tool-joint surfaces, which will certainly affect the friction between the two. However, there is no general, unambiguous relationship between the two. High friction and low wear rate can occur simultaneously (e.g., in a brake shoe) as can the reverse (e.g., metal cutting with a cutting oil). This means that wear and friction have to be evaluated separately. The API method for evaluating the effect of mud composition on the friction between the casing and the tool joint uses the lubricity coefficient instrument, which is a small-scale laboratory test machine. No standard method is available for evaluating casing wear. Initially, both the API lubricity tester and several other small-scale test machines were used to measure friction and/or wear in steel/steel contact areas immersed in various muds. Wear and friction were measured later with full-scale equipment. The effects of mud density, type of weighting material, and the addition of several mud additives and salts, lubricants and diesel, and silt and sand were investigated in water-based muds. For comparison, several tests were also done with standard invert-oil-emulsion

Summary Problems with bentonite quality and bentonite-quality-control methods have consistently been observed in the field in recent years. These problems are investigated and proposals are made on how to mitigate them. A number of commercial bentonites have been analyzed and their performances evaluated (1) to determine to what extent quality variations performances evaluated (1) to determine to what extent quality variations occur between commercial bentonites from different manufacturers, (2) to determine the cause(s) of these variations, and (3) to develop a proper quality-control method for bentonite. The results show significant variations in quality between Oil Companies Materials Assn, (OCMA) and API grade bentonite. The main cause of these variations is the different type and amount of extending chemical added. Differences in rheology and fluid loss between bentonite suspensions in fresh water, seawater, and hydrogen-peroxide (H2O2) solutions give a quantitative measure of bentonite extension. Differences in performance before and after hot-rolling do the same. Introduction Commercial bentonite (sodium montmorillonite) is used worldwide as a drilling-fluid additive. Its main functions are to viscosity the mud and to reduce fluid loss to the formation. We have noticed a general decline in bentonite quality over the past years that is reflected not so much in noncompliance with OCMA or API specifications, but rather in an unpredictable performance in a number of applications:bentonite properties change after longterm storage;addition of bentonite to potassium-chloride (KCl)/polymer muds is ineffective in building potassium-chloride (KCl)/polymer muds is ineffective in building viscosity;,flocculation of cement slurries occurs in cases where bentonite is used as an extender;properties of bentonites change significantly from one batch to properties of bentonites change significantly from one batch to another; andpoor and/or unpredictable performance of bentonite has been reported for drilling under harsh conditions. As a result of these observations, an investigation mainly started to determine whether significant quality variations occur between commercial bentonites and, if so, to determine their nature, and to develop a proper bentonite quality-control method.

Shale hydration tests have been carried out at confining pressures, ranging from 0 to 350 bar. The results show a high swelling pressure, without significant expansion, at high confining pressure and a large expansion without swelling pressure in the unconfined situation. It is also shown that different salts have different effects on swelling and that the effect of mud additives other than salts is negligible. The test results indicate that, for the shales tested, water structure modification and not ion exchange is principally responsible for the inhibitive effect of salts. In addition, borehole erosion and cutting disintegration tests have been carried out. The results of these tests show a significant effect of various polymeric mud additives in terms of cohesion.

Summary Microseismic events or acoustic emissions associated with hydraulicfracturing are recorded with a borehole seismic tool in a deviated well duringmultirate injection, shut-in, and flowback. The event locations indicate thatfracture orientation, length, and height are compatible with regional stressdirections and estimates of the fracture size that are based on pressuredecline. Introduction Fractures, either natural or induced, can be the most important featurecontrolling the productivity of a hydrocarbon reservoir. Knowledge of theirorientation, length, and height can affect all stages of planning and isespecially important for horizontal drilling, stimulation of low-permeabilityreservoirs, in-fill drilling, and EOR programs. Many methods have beendeveloped to model or measure the characteristics of hydraulically inducedfractures. These include insitu or regional stress studies, inference ofmaterial-strength properties from wireline logs, pressure-decline analysis, properties from wireline logs, pressure-decline analysis, laboratoryexperiments on cores, and comparison of such pre- and postfracture logs astemperature, borehole images, gamma ray, and postfracture logs as temperature, borehole images, gamma ray, and sonic. Most of these techniques investigate thefracture in the near-wellbore region and are successful only in open holescontained in the fracture plane. The acoustic-emission-mapping technique wasdeveloped for applications in fracturing crystalline rock and has a history ofsuccess in such hot-dry-rock geothermal fields as Fenton Hill, Camborne, and Kakkonda. Acoustic emissions have been used with varying degrees of success tomap hydraulic fractures in sedimentary formations. The high attenuation ofseismic waves in sedimentary rocks imposes limitations in the instrumentationand acquisition configurations that can "listen" to fractures. Acousticemissions may be thought of as microearthquakes, or microseismic events causedby brittle fracture in a region surrounding a hydraulic fracture. They arebelieved to be caused by a combination of stress release in the zone of highpore pressure surrounding the fracture and the presence of inhomogeneities inthe formation. Laboratory presence of inhomogeneities in the formation. Laboratory experiments show that these events arise from discrete ruptures onthe fracture plane. 10 If these microseismic events can be located, the strikeand dip of the fracture can be determined. This provides a characterization ofthe fracture far from the borehole provides a characterization of the fracturefar from the borehole that can be implemented in cased or nonvertical wells, even for nonvertical fractures. As in earthquake location, acoustic-emissionlocation is more accurate when multiple receivers are used. The ideal situationis to deploy several vertical arrays of three-component sensors near theexpected fracture to get accurate locations of many events by triangulation. Unfortunately, few hydrocarbon-reservoir sites present this opportunity. Inprinciple, however, locations can be present this opportunity. In principle, however, locations can be determined with a single calibrated three-componentsensor, either in the injection well itself or in a nearby observation well. Assuming that the first motion created by the source mechanism is radiallycompressional (or dilatational), the polarization angle of the event's firstarrival compression al (P) wave defines the direction from receiver to event. This is a simple problem to solve in a homogeneous medium but is slightly moredifficult in a layered medium because the seismic rays bend at interfaces. Thedifference in travel times between the P wave and the secondary, or shear (S)wave is related to the distance to the event. This location method requiresknowledge of the P- and S-wave velocity structure near the fracture and carefulplacement of the sensor to ensure that all events occur above or below it. Thelatter requirement arises because the source mechanism is unknown: i.e., a Pwave coming from above has the same signature as a dilatational wave frombelow. The first requirement, knowledge of the velocities, is often satisfiedonly in an approximate fashion: average velocities from prestimulation soniclogging, check shots, or vertical seismic profiles (VSP's) can give reasonableestimates, and poststimulation profiles (VSP's) can give reasonable estimates, and poststimulation velocities may be slightly different. Green and Bariareported a decrease of less than 1 % in P-wave velocities after injectiontreatment in crystalline rock. Deriving the polarization information fromseismic records requires tool calibration because the polarization calculationdepends on the relative amplitudes recorded on the three axes. Calibration canbe done in situ by clamping the tool at the level where the fracture recordingwill occur and locating a known energy source, such as an airgun shot, a buriedcharge, or a downhole source, in a nearby well. Fracture-Mapping Survey Objective. The objective of the experiment was to test the feasibility of an EOR program that would entail water injection above fracturing pressure if wecould confirm that the fracture would not grow toward the pressure if we couldconfirm that the fracture would not grow toward the producing well. Bymonitoring the seismic activity generated by high producing well. By monitoringthe seismic activity generated by high rate injection, the fracture orientationand propagation length can be determined. Data Acquisition. The downhole equipment consisted of a modified slimholeseismic tool with a 2.125-in. OD and a 20,000-psi pressure rating. The lockingdevice was a hydraulic arm. The modification involved replacing the triaxialgeophone assembly with accelerometers so that we could record signals in therange of thousands of cycles/sec. The laboratory-calibrated accelerometers hada sensitivity of 1,000 X 10 coulomb per unit gravitational acceleration with alinear response of more than 2,000 cycles/sec over a temperature range up to482F. These sensors were arranged orthogonally with the z axis tangential tothe tool axis and the x axis in the same plane as the clamping arm, effectivelythe radial component. The relative hearing of the tool is given by the anglebetween the x axis and the vertical plane along the deviated borehole. Thisangle was measured by a circular potmeter attached to the tool housing, Thewiper of the potmeter always points vertically because of an attached weight. During the survey, the × axis was at an angle of 27 to the vertical (Fig. 1, insert). Surface airgun sources were deployed to obtain an independentmeasurement of the tool orientation, effectively calibrating the relativeresponse of the tool's three axes at the frequencies contained in the airgunsignature. The surface shots, however, were not detected by the downholeaccelerometers, probably because of the low-frequency content of the signalspropagating from the source through soft sedimentary material. Because thesurrounding area had a heavy surface buildup, it was not feasible to attemptother dangerous or destructive calibration techniques so we base ourcalibration on the laboratory accelerometer response. Surface equipment wassupplied by a subcontractor and consisted of amplifiers, a videocassetterecorder, and a power supply for the downhole tool. Data were recorded inanalog form on four 240-minute videocassette tapes, and the downhole signalswere monitored on an oscilloscope during the survey. SPEFE P. 139