--> Laboratory Compaction Behavior of Siliciclastic and Carbonate Reservoirs: The Impact of Heterogeneity and What are the Differences?

AAPG Middle East Region Geoscience Technology Workshop

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Laboratory Compaction Behavior of Siliciclastic and Carbonate Reservoirs: The Impact of Heterogeneity and What are the Differences?

Abstract

Reservoir compaction of siliciclastic sediments and specifically weakly consolidated sands is largely controlled by grain crushing resulting from stress concentrations at the grain level and controlled by grain-scale fabric. Microscopically, grain-scale failure results from high stress sensitivity to small changes in confining pressures, stress path, and rate of loading. For textures with rounded grains or planar grain contacts, the occurrence of lower stress concentrations limits grain crushing. In these cases, compressibility is small, and the resulting deformation behavior is nearly elastic. Conventional carbonate compaction behavior is more aligned with depositional fabric and bimodal porosity systems and commonly exhibits plastic deformation. Pore volume loss occurs from compacting pores and under extreme deformation, plastic pore collapse. In contrast, pore volume reduction observed in clastic sediments is produced from grain splitting infilling the original porosity. Regardless, the changes resulting from pore collapse (carbonates) or grain crushing (clastic sediments) are permanent and can significantly reduce production. Although the compaction phenomenon is different between carbonate and clastic reservoirs, similar laboratory testing techniques can be applied to assess compaction behavior from preproduction conditions through to abandonment. During depletion, the reduction in reservoir pressure results in unequal increases in vertical and horizontal effective stresses and thus an overall increase in the effective mean and shear stresses on the reservoir. At reservoir pressures below a critical value (obtained via laboratory testing or post-failure field analysis), the reservoir may compact at accelerated rates. In this study, a series of tests were designed to probe all possible depletion scenarios to compare behavior between regional carbonates and siliciclastic rocks. Rock failure parameters were evaluated through a sequence of tests. Failure envelopes defining shear (dilatant) and compaction (“cap”) for compactable sediments are often strongly nonlinear. For field applications, it is useful to provide a visualization of the preproduction-state insitu stress conditions and the possible stress path trajectories of the reservoir (from triaxial Ko = 0 to hydrostatic Ko = 1) as a function of reservoir depletion. Using this display, the level of depletion resulting from accelerated compaction was identified through laboratory testing. Tests conducted for assessment of reservoir compaction are uniaxial-strain compression (far-field compaction), triaxial compression (near-wellbore compaction), hydrostatic (define the compactant cap), and constant stress path (fixed Ko, far-field compaction). Geomechanics laboratory studies should always be complemented with mineralogical and petrographic studies of the samples before and after compaction to evaluate changes at the grain scale and the impact of rock texture on compaction and general rock properties. Testing conducted on both clastic and carbonate intervals was designed to capture all possible depletion scenarios during the potential life of the reservoir. Compaction characteristics for carbonates have shown that rock with porosity >28% has a propensity for accelerated compaction prior to planned abandonment pressures. Further, accelerated compaction does not occur for many carbonates with porosities below 25% at maximum depleted conditions. The compressibility of clastic sands is largely controlled by their granular texture and stress concentrations at the grain level. Results from sandstone samples with different textures and grain-to-grain relationships provide a visualization of the overall behavior. This is represented with three fundamental regimes: 1) Low compressibility and predominantly elastic behavior, 2) intermediate initial compressibility and subsequent strain-softening with depletion, and 3) high initial compressibility and subsequent strain-hardening with depletion. This presentation outlines core analysis workflows that can adequately assess potential changes to reservoirs during depletion—from preproduction conditions to abandonment. These results are then integrated with reservoir simulators for long-term reservoir management. Further, the presentation highlights the importance of understanding rock heterogeneity prior to initiating any core analysis program.