SUMMARYWater-absorbing rocks are formed from minerals that can hold water in their crystal structure or between grain boundaries. Such water absorption is often accompanied by a change in the crystar dimension that manifests itself as a swelling of the rock. Swelling is particularly pronounced in rocks containing phyllosilicates because of the ease with which these minerals hydrate; it is thus of geological and geotechnical relevance in shales, clay-rich soils and zeolitized tuffs. The model of hydration swelling that we present here is based on extended versions of the equations of poroelasticity and Darcy's transport law, which we derive using a non-equilibrium thermodynamics approach. Our equations account for the hydration reaction under the assumption that the reaction rate is fast in comparison with the rate at which hydraulic state changes are communicated through the rock, i.e. that local physico-chemical equilibrium persists. Using a finite-element scheme for solving numerically the governing equations of our model, we simulate the creep of shales during a routine swelling test and calculate the stress and strain distributions around wellbores drilled in shale formations that undergo swelling. We show that swelling effects promote tensile failure of the wellbore wall.
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 Perforation pressure drop and its decrease caused by perforation erosion during a hydraulic-fracturing treatment are critical factors that need to be considered in treatment design, particularly when the limited-entry technique is implemented along multiple perforation clusters to ensure more-uniform fluid distribution. The simultaneous increases in the discharge coefficient Cd and perforation diameter D during perforation erosion require consideration of the temporal changes of these two variables to characterize the perforation-erosion behavior. In this paper, we present a perforation-erosion model dependent on abrasion mechanisms and the procedure to determine the specific erosion parameters that can be corroborated from laboratory data. Our modeling results demonstrate that it is inappropriate to assume an alternate increase in Cd and D, as considered in some conventional correlations. Once the erosion parameters are empirically inferred, we incorporate our model into a nonplanar hydraulic-fracturing simulator to determine appropriate perforation-number distributions at different clusters to ensure a successful limited-entry treatment that generates relatively even fluid distribution and uniform fractures.
The technology of multiple hydraulic fracture stimulation in horizontal wells has transformed the business of oil and gas exploitation from extremely tight, unconventional hydrocarbon bearing rock formations. The fracture stimulation process typically involves placing multiple fractures stage-by-stage along the horizontal well using diverse well completion technologies. The effective design of such massive fracture stimulation requires an understanding of how multiple hydraulic fractures would grow and interact with each other in heterogeneous formations. This is especially challenging as the interaction of these fractures are subject to the dynamic process of subsurface geomechanical stress changes induced by the fracture treatment itself. This paper consists of two parts. Firstly, an idealised analytical model is used to highlight some key features of multiple hydraulic fractures interaction, and to provide a quantification of 'stress shadow'. Secondly, a new non-planar three dimensional (3D) hydraulic fracturing numerical model is used to provide an insight into the growth of multiple fractures under the influence of subsurface geomechanical stress shadows. Attention is given to studying the height growth of multiple fractures.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.