Summary
We present a novel approach that combines dynamic reservoir simulations and special core tests to model the extent of invasive damage and its impact on flowback during production. A radially adaptive 3D microsimulator is used to estimate the extent and impact of filtrate invasion on near-wellbore saturation and reservoir pressure. Time-varying reservoir exposure is used to simulate the acts of drilling, tripping, completions, and workovers. Extremely fine, core-scale grids are used to capture saturation and pressure in the invasion zone. Special core tests using a specially designed core holder are conducted on the subject reservoir core. Test results are interpreted to obtain an estimate of endpoint relative permeabilities, dynamic mudcake effect on filtrate loss, and impact of solids invasion on return permeability. The saturation and pressure profiles from this model are then used as initial conditions in a sector-scale simulator to model flowback effects. Absolute-permeability damage is modeled using the core-test results as an incremental and hyperbolically recovering effect during flowback simulations. A near-wellbore fine-grid overlay is used to capture the near-wellbore effects from the microsimulator results. Several sensitivities, including initial reservoir pressure, degree of overbalance and drawdown, heterogeneity, anisotropy, and mudcake effect, are examined. Equivalent skin factors that vary with time and depth are developed to enable comparison with full-field simulations. A horizontal-well example is used to illustrate the results of the study. Results illustrate the stark and often underappreciated effects of invasive damage on flowback and, therefore, on production performance. The methods described in this work can be used in reservoir-specific studies to quantify formation damage and aid in the selection of mud types, drilling techniques, and remediation methods required to improve performance. It is hoped that this work bridges the typically empirical damage-characterization methods and dynamic reservoir simulations.
Introduction
Conventional (or overbalanced) drilling and workover operations invariably result in invasion of filtrate and solids present in the drilling and workover fluids. In most cases, the damage caused is limited to a near-wellbore region and can reduce productivity because of degradation in effective permeability. Permeability degradation from filtrate and solids invasion could be caused by a variety of damage mechanisms, such as blockage of pore throats by solids, reduction in relative permeability to hydrocarbons because of a change in saturation, phase blockage, and clay swelling in the formation. Damage can be harsher in horizontal wells and mature reservoirs because of greater overbalance and longer duration of exposure to drilling fluids.
During drilling, mudcake buildup can reduce the invasion depth. The buildup and effectiveness of mudcake depend greatly upon the formulation of the mud, the type and heterogeneity of the formation being drilled, the maturity of the reservoir, and the degree of overbalance during drilling or workovers. In horizontal wells, mudcake effectiveness is compromised further because of repeated movement of the pipe against the mudcake, leading to several events of removal and re-laying of the mudcake. The effects of damage also can be alleviated by the use of remedial stimulation techniques such as acidizing and hydraulic fracturing. These may not always produce the desired results, particularly in horizontal wells in highly heterogeneous formations. Moreover, implementing some of these techniques in horizontal wells is difficult.
Given the potential for reduced productivity from invasion, characterization of invasion-induced damage has been of interest for decades. However, the implicit presumption when dealing with invasion-induced damage has been that it can be mitigated (by appropriate selection of muds and formation of mudcake), bypassed (through perforations), or remedied (through stimulation and fracturing).
Most prior damage-characterization work has been empirical in nature, relying on log and core tests to assess damage parameters. More recently, some authors also have attempted to quantify and model formation damage from the fundamental principles of deep-bed filtration, fines migration, and percolation theory. Dynamic modeling of invasion with numerical simulations has also received much-needed attention in recent times. However, much of the numerical invasion-modeling work in the literature has focused on the invasion only (typically because of interest in the impact of the invasion zone on log accuracy), and very few works have dealt with the impact of invasion on flowback during production. The problem of bridging empirical models and dynamic simulations to obtain reasonable estimates of the impact on production has been one of the challenges.
In this work, we present a novel approach that combines dynamic reservoir simulations and special core tests to model the extent of invasive damage and its impact on flowback during production. The approach uses an ultrafine-grid numerical simulator to model invasion, with parameters calibrated to special core tests. Flowback is then modeled using a sector-scale simulator with near-wellbore fine gridding, with the initial saturation and pressure profiles as determined by the invasion model and parameters calibrated to the core tests. The experimental and numerical approaches are described in detail, along with examples to illustrate the use of the methods we describe. Several sensitivity analyses are presented to demonstrate the often overlooked and underestimated impact of invasion on productivity. The method can be used to compare different mud types, evaluate the benefits of different remediation methods, and value the impact of underbalanced drilling (UBD) on productivity.
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