Summary Open literature and new experimental compaction data from five reservoir and 16 outcrop sandstones are used to delineate the near-elastic, inelastic, and failure domains in 3D-stress space for porosity classes of 5 to 15%, 15 to 25%, and 25 to 35%. Applications of this compaction-domain model include the analysis of the extent of the near-elastic domain (where elasticity theory can be used to describe and predict rock deformation), the pore-volume compressibility (Cpp), and the permeability reduction as a function of reservoir stress path. This is illustrated for a well-consolidated sandstone reservoir with an average porosity of approximately 18%. Two aspects of dynamic reservoir modeling in the near-elastic domain are addressed: calculation of Cpp from raw volumetric-compaction data as a function of isotropic total stress change, and the correction of Cpp for a nonhydrostatic reservoir stress path. Open-literature work combined with our experimental data indicates that the compaction-induced permeability reduction of 15 to 25% porosity sandstone in the near-elastic domain depends predominantly on the increase of the effective mean stress, not on the reservoir stress path.
Long-term stability of horizontal wellbore completions with uncemented liners in weakly consolidated to unconsolidated sandstone formations (e.g. Gulf of Mexico, Nigeria) remains an area of concern. This paper presents the results of dedicated polyaxial cell laboratory experiments addressing this issue. In addition, the influence of rock failure in the near-wellbore region on well productivity was studied. Large blocks of a weak artificial sandstone were prepared. A hole was drilled in these blocks, and production conditions at various values of in-situ stress, drawdown and watercut both in the absence and presence of a liner, were simulated. During testing, the hole was kept at a horizontal position in order to realistically simulate the influence of gravity forces on the movement of sand debris. The process of hole failure and restabilisation was continuously monitored by an endoscope coupled to a videocamera. The experimental results show that in the presence of a slotted liner, and in the absence of watercut, rock failure leads to a gradual annulus fill-up with loose sand, eventually resulting in a stable configuration in which only a small fraction of the farfield stresses is transferred to the liner. These results are further supported by elasto-plastic calculations. Rock failure around the liner is shown to have only a minor effect on productivity. This result implies that rock failure around uncemented liner completions will generally not be noticed at the wellhead. The introduction of a small (<5%) watercut resulted in massive sand production and subsequent liner collapse. This can be explained by the fact that watercut destroys capillary cohesion, thereby destabilising sand arches over the slots. Introduction Reliable predictions of sand production potential are required to make realistic sand production management and contingency planning possible. Unnecessary application of sand exclusion measures results in increased completion costs and considerable loss of well productivities. Further, sand prediction may assist in selecting the most attractive sand control techniques. Over the years, a large number of models for sand production prediction have been developed, see e.g. Refs. 2-9. These models generally focussed on the prediction of the onset of sand production. A new conceptual model for initial sand production prediction is presented in an accompanying paper. However, in many situations a certain degree of sand failure around the wellbore, and resulting sand production, is acceptable within limits. This paper presents laboratory experiments focusing on re-stabilisation after initial sand failure around horizontal wellbores with or without uncemented liners, in weakly consolidated to unconsolidated sandstones. Long-term stability and productivity of these completion types remains an area of concern in many fields, e.g. Gulf of Mexico, Nigeria, North Sea, etc. Large blocks of a weakly consolidated (0.46 MPa cohesion) artificial sandstone were prepared, and a hole was drilled in these blocks. The effects of in-situ stress, flow rate (drawdown), watercut, and completion type (open hole, predrilled liner, slotted liner) on stability and productivity were investigated. During testing, the hole was kept at a horizontal position in order to realistically simulate the influence of gravity forces on the movement of sand debris. The process of hole failure and re-stabilisation was continuously monitored by an endoscope coupled to a videocamera. The liners used in the tests were made out of (transparent) plexiglass in order to monitor the failure process around the liner with the endoscope. The experimental results show that in the absence of a liner, increasing the far-field effective stress leads to gradual hole closure instead of a sharply defined 'failure'. In the presence of a slotted liner, and in the absence of watercut the endoscope images showed a gradual annulus fill-up by loose sand with increasing far-field stress. P. 35
Summary Long-term stability of horizontal wellbore completions with uncemented liners in weakly consolidated to unconsolidated sandstone formations (e.g., the Gulf of Mexico and Nigeria) remains an area of concern. In this paper we present the results of dedicated polyaxial cell laboratory experiments that address this issue. In addition, the influence of rock failure in the near-wellbore region on well productivity was studied. Large blocks of a weak artificial sandstone were prepared. A hole was drilled in these blocks, and production conditions at various values of in-situ stress, drawdown and water cut, both in the absence and presence of a liner, were simulated. During testing, the hole was kept at a horizontal position in order to realistically simulate the influence of gravity forces on the movement of sand debris. The process of hole failure and re-stabilization was continuously monitored by an endoscope coupled to a video camera. The experimental results show that in the presence of a slotted liner, and in the absence of a water cut, rock failure leads to gradual annulus fillup with loose sand, eventually resulting in a stable configuration in which only a small fraction of the far-field stresses is transferred to the liner. These results are further supported by elasto-plastic calculations. Rock failure around the liner is shown to have only a minor effect on productivity. This result implies that rock failure around uncemented liner completions will generally not be noticed at the wellhead. The introduction of a small (<5%) water cut resulted in massive sand production and subsequent liner collapse. This can be explained by the fact that a water cut destroys capillary cohesion, thereby destabilizing sand arches over the slots. Introduction Reliable predictions of sand production potential are required to make realistic sand production management and contingency planning possible. Unnecessary application of sand exclusion measures results in increased completion costs and considerable loss of well productivity. Further, sand prediction may assist in selecting the most attractive sand control techniques. 1 Over the years, a large number of models for sand production prediction have been developed; see, for example, (Refs. 2-9). These models generally focused on the prediction of the onset of sand production. A new conceptual model for an initial sand production prediction is presented in another paper.10 However, in many situations a certain degree of sand failure around the wellbore, and resulting sand production, is acceptable within limits. In this paper we present laboratory experiments focusing on re-stabilization after initial sand failure around horizontal wellbores, with or without uncemented liners, in weakly consolidated to unconsolidated sandstones. Long-term stability and productivity of these completion types remains an area of concern in many fields, e.g., the Gulf of Mexico, Nigeria, North Sea, etc. Large blocks of a weakly consolidated (0.46 MPa cohesion) artificial sandstone were prepared, and a hole was drilled in these blocks. The effects of in-situ stress, flow rate (drawdown), water cut, and completion type (openhole, pre-drilled liner, slotted liner) on stability and productivity were investigated. During testing, the hole was kept at a horizontal position to realistically simulate the influence of gravity forces on the movement of sand debris. The process of hole failure and re-stabilization was continuously monitored by an endoscope coupled to a video camera. The liners used in the tests were made of (transparent) plexiglass in order to monitor the failure process around the liner with the endoscope. The experimental results show that, in the absence of a liner, increasing the far-field effective stress leads to gradual hole closure instead of a sharply defined "failure." In the presence of a slotted liner, and in the absence of a water cut, the endoscope images showed gradual annulus fillup by loose sand with increasing far-field stress. Finally, a stable configuration was achieved in which the liner was completely surrounded by loose, unproduced sand and plastically deformed sandstone, as was also confirmed by post-test computer tomography (CT) scans. This configuration was still stable at a vertical effective stress equal to three times the measured collapse strength of the liner. These observations were further supported by elasto-plastic calculations, which showed that redistribution of stresses around the hole resulted in forces on the liner which were only 5 to 25% those of the far-field effective stresses. However, the introduction of only a small (5%) water cut was sufficient to disturb the above-mentioned stable configuration, and resulted in massive sand production and subsequent liner collapse. This can be explained by the fact that the water cut destroys capillary cohesion, thereby destabilizing sand arches over the slots. Post-test CT scans showed large, horizontally oriented, washouts adjacent to the wellbore. The resulting stress concentrations on the top and bottom of the liner finally caused its collapse. The experiments showed that, in the absence of a water cut, rock failure around the liner only results in a small change in productivity. These results were further confirmed by elasto-plastic calculations. One implication is that no observed productivity decrease in the field does not mean that there is no rock failure, which may have consequences, e.g., for future water breakthrough (sand production) and for selective placement in stimulation/shutoff. In the next section we provide a description of the laboratory setup and the experiments that were conducted. Then we present and interpret the experimental results. In the last section conclusions are drawn. Experimental Approach A total of six polyaxial cell tests were performed on low-strength artificial sandstone blocks (26.25 cm×26.25 cm×38 cm) with 25.4 mm diameter horizontal holes. Artificial sandstone was used rather than a natural poorly consolidated rock (such as Saltwash South) since natural low-strength rocks have frequently been observed to have highly variable mechanical properties and physical characteristics. An artificial rock with consistent properties was fabricated using a stringently controlled manufacturing program.
Summary Conventional theories of sand production prediction distinguish between compressive (shear) cavity (perforation, borehole) failure, induced by a combination of in-situ stress and drawdown, and tensile cavity failure, induced by the near-cavity pore pressure gradient. In this paper we show, using a global criterion for strain localization around the cavity (i.e., cavity failure), that in most cases the preference for either compressive or tensile cavity failure only depends on the cavity size and on the constitutive properties of the rock, and not on effective near-cavity stress or the pore pressure gradient. It is shown that "large" cavities (e.g., boreholes) always fail in compression rather than in tension failure. Only for sufficiently small cavities (e.g., perforations) in weak, moist sandstones, is compressive failure supressed and tensile failure possible. The above results deviate significantly from previous sand production prediction concepts. They are further supported by laboratory sand production experiments with large cavities, in which sand failure was only observed for near-cavity effective stresses above a certain threshold, independent of applied drawdown (i.e., flow rate). The experiments exhibited no correlation between the (non) occurrence of sand production and drawdown. From these results, it may be concluded that the primary role of fluid flow in sand production is the transport of loose sand (debris) resulting from compressive failure, rather than failure of the intact sandstone itself. Introduction Both the effective stress near the wellbore and flow rate into the wellbore have long been identified as the main formation destabilizing factors influencing sand production. Several sand production prediction methods have been proposed based on these parameters. The Drucker-Prager model proposed by Antheunis et al.1 and Mohr-Coulomb model proposed by Coates and Denoo2 are examples of such methods based on consideration of the intact rock compressive strength, drawdown and in-situ stress. Alternatively, models such as those of Bratli et al.3,4 and of Perkins and Weingarten5,6 compare the flow-induced pressure gradient with the residual strength of disaggregated material surrounding the cavity (perforation, borehole). A well-accepted conceptual model for sand production was proposed by Morita et al.7,8In that model, sand production can either be triggered by compressive (shear) failure, induced by a combination of in-situ stress and drawdown, or by tensile failure, induced by the near-cavity pore pressure gradient. Compressive failure around a cylindrical cavity leads to the formation of breakouts adjacent to the cavity. Whether compressive failure or tensile failure prevails will depend on the precise values of the in-situ stress, drawdown and flow rate in relation to the rock strength. The extreme condition of compressive failure at zero flow rate is analogous to the problem of hollow cylinder collapse.1,9,10 The other extreme, tensile failure at zero (or very low) near-cavity effective stress, would be similar to the unconsolidated sand failure experiments performed by Hall and Harrisberger11 or by Bratli and Risnes.3 In this paper we present the results of theoretical and experimental studies of the prediction of sand failure around cylindrical and hemispherical cavities in weak sandstones under a variety of in-situ stress and flow conditions. The theoretical analysis uses a criterion for global strain localization to define macroscopic failure around the cavity. The analysis is based on bifurcation theory in combination with a Cosserat continuum. The latter accounts for effects of an internal length scale of the rock (for example, grain size). The constitutive model used in this analysis was calibrated to conventional triaxial compression test data. Previously, this approach was successfully used to describe, amongst others, the process of strain localization resulting in breakout formation around axisymmetrically loaded boreholes,12 and to reproduce the experimentally observed size dependency of hollow cylinder strength for the friable Red Wilmore13,14 and Castlegate15,16 sandstones. In the present theoretical and experimental studies, both in-situ stress and flow rate (i.e., the near-cavity pore pressure gradient) were varied over a large range of values in order to induce either compressive or tensile sand failure. The theoretical study also addressed the influence of cavity size on compressive vs. tensile failure. Finally, different flow conditions (laminar vs. turbulent, steady-state vs. early transient) were also addressed. The theoretical results show that in most cases the "preference" of a cavity for either compressive failure or tensile failure depends only on the cavity size, and not on the near-wellbore effective stress or fluid flow rate. In particular, it is shown that large cavities (e.g., boreholes) always fail in compression, whereas small cavities (e.g., perforations) may either fail in compression or in tension, depending on the material properties and moisture content (oven-dry vs. moist rocks). Only in cases of an extremely localized near-wellbore pressure gradient (e.g., very early transient flow, permeability damage), may "medium-size" cavities also exhibit tensile failure at high pressure gradient values. The above concept is entirely different from previous sand failure concepts (e.g., those of Morita et al.7,8), and is further supported by the laboratory sand production experiments (using large cavities), in which sand failure was only observed for near-cavity effective stresses above a certain threshold, independent of applied drawdown (i.e., flow rate). The experiments exhibited no correlation between the (non) occurrence of sand production and drawdown. Our theoretical results also provide an explanation for the observed high "apparent" hollow cylinder strength [in relation to unconfined compressive strength (UCS)], which is particularly pronounced in softer sandstones. Conventional explanations of this phenomenon are based on a stress-dependent Young's modulus.17,18 However, our analysis shows that the apparent high hollow cylinder strength can be largely ascribed to a size effect. Finally, our results provide an analytical framework for estimating an upper bound to the observed size dependency of hollow cylinder strength.13,19-22
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