The Effect of Small, Discontinuous Shales on Oil Recovery

Abstract

A mathematical model is presented for analyzing the effect of small, discontinuous shales on oil recovery. The model was validated by calculations with a fine-grid computer model. The study illustrates the importance of teamwork between geologists and engineers when describing a reservoir and when predicting its performance. Introduction A complex problem when predicting reservoir performance is assessment of permeability of a formation performance is assessment of permeability of a formation perpendicular to bedding planes. Vertical permeabilities, perpendicular to bedding planes. Vertical permeabilities, which may be less than horizontal permeabilities because of orientation of rock particles and cementing material, can be measured on core samples. It also is necessary to determine distribution of nonpay intervals because small amounts of impermeable rock can profoundly affect vertical permeabilities, even if the rock is discontinuous and randomly distributed. Lateral dimensions of shale laminae, shale bodies, or other impervious materials and their distribution are difficult to describe solely from data on cores and logs. Proper description requires knowledge of depositional environment and detailed information from geologic studies of outcrops and recent sediments. Correct assessment of vertical permeability is particularly important in gas drive operations because high oil particularly important in gas drive operations because high oil recovery depends on effective oil drainage by gravity. For example, shale bodies may be large enough laterally in some reservoirs to prevent coning and yet be few enough to permit good vertical oil drainage. A laminate barrier in a pinnacle reef (carbonate reservoir) was extensive enough to cause loss of miscibility by collecting oil and part of an LPG bank above it. Recent studies indicate the presence of frequent, small, randomly distributed shale bodies in the upper formation of a large sandstone reservoir, which reduce vertical permeability in that interval. Fortunately, conceptual models based on geologic studies of recent sediments, outcrops, cores, and logs are available for describing the distribution and dimensions of pay and nonpay in both sandstone and carbonate reservoirs. Also, reservoir simulators are available for solving increasingly complex problems in practical times at reasonable costs. Construction of simulators is relatively straightforward when a reservoir is divided into separate zones by continuous barriers. Usually, at least three layers are provided in a computer model for each zone. Thin layers provided in a computer model for each zone. Thin layers are used above impervious boundaries to represent flow from gas-invaded regions. An alternative procedure is to generate pseudorelative permeabilities for layers above boundaries. Layers beneath impervious boundaries in water-invaded regions require similar treatment for accurate simulation of reservoir performance. However, no standard treatment is available for modeling reservoirs containing many small, discontinuous barriers. Adequate simulation of performance in large regions of a reservoir could requite many more small computer blocks than the user could afford. This paper illustrates how geologists and engineers can cooperate in describing and predicting oil recovery from complex distributions of permeable and impermeable intervals. A brief overview is given on geologic modeling of different types of deposits. A simple mathematical model is presented for assessing the impact of a given sized impervious body on oil recovery. JPT P. 1531

Analysis of Heavy Oil Recovery by Thermal EOR in a Meander Belt: From Geological to Reservoir Modeling

“…The coarsest deposits occur as elongate bodies parallel to the channel belt axis when channel bends migrate. These depositional units and associated shale drapes can influence reservoir behavior (Swanson, 1993) because they are potential baffles and barriers to fluid flow (Richardson et al, 1978;HartkampBakker and Donselaar, 1993). 8 000 1 000 2 000 3 000 4 000 5 000 6 000 7 000 8 000…”

“…Many studies have already explored geological modeling of various scales of heterogeneities within fluvial reservoirs (process based, deterministic and stochastic). Many of these studies investigate stratigraphic architecture (sand and shale body dimensions and distribution) and reservoir connectivity and heterogeneity assessment at the reservoir scale (Richardson et al, 1978). Other studies focused on the potential effect on fluid flow of internal sand bodies and shale heterogeneities (Hartkamp-Bakker and Donselaar, 1993;Lerat et al, 2010).…”

“…Because of the high variability of these deposits, the size of the heterogeneities are commonly smaller than the typical well spacing in developed fields. These depositional units and associated shale drapes can influence reservoir behavior (Swanson, 1993) because they are potential baffles and barriers to fluid flow (Richardson et al, 1978;Hartkamp-Bakker and Donselaar, 1993). In addition, internal facies variations, such as the vertical change from trough cross-bedded to ripple-laminated sandstone and silty shales, produce a fining-upward trend in grain-size and a corresponding decrease in porosity and permeability.…”

Characterizing the Role of Shale Geometry and Connate Water Saturation on Performance of Polymer Flooding in Heavy Oil Reservoirs: Experimental Observations and Numerical Simulations

“…These shale streaks cannot be correlated between wells, and their likely location within the reservoir must be predicted using stochastic techniques conditioned to existing core and outcrop data (Muggeridge et al 2002). Randomly distributed, discontinuous barriers can cause a significant reduction in the vertical transmissibility due to the generation of the tortuous path for the fluid movement (Richardson et al 1978). Consequently, the presence of these discontinuous shale streaks can decrease the effective single-phase permeability (Muggeridge et al 2005).…”

“…However, the effect of these heterogeneities on two-phase fluid flow is not well-understood. During immiscible flooding, oil in the vicinity of isolated shale is initially bypassed by the displacing fluid, and subsequently drains from around the shale driven by viscous and gravitational forces (Richardson et al 1978;Davies and Haldorsen 1987;Thomas 1990;Jones et al 1994). The initial bypassing of the oil leads to the premature breakthrough of displacing fluids.…”