Detection and Location of Two Parallel Sealing Faults Around a Well

Abstract

A new technique is presented for detecting the existence of and distance to two parallel sealing boundaries surrounding a producing oil or gas well. A log-log plot of time rate of change of well pressure vs. time provides a unique behavior to detect such a condition and allows the use of type-curve matching techniques to determine the khproduct. Introduction The idea of detecting and locating a reservoir boundary from transient pressure-time data first appeared in 1951. In that year Horner1 presented the transient pressure behavior of a constant-rate well located near a linear sealing fault. He also presented a method to calculate the distance to the fault from buidup pressure data. Dolan et al.2 applied the technique to drillstem tests. Davis and Hawkins3 gave an equation to determine the distance to the fault from drawdown pressure data. Gray4 reviewed these methods and discussed their limitations. Bixel et al.5considered a more general problem: a well located close to a linear discontinuity across which hydraulic diffusivity changes. They showed a procedure to determine the distance to such a discontinuity. Evrenos and Rejda6 employed superposition techniques to simulate various combinations of linear boundaries of interest in gas storage systems and showed how a match between the actual test data with various hypothesized model conditions can be used to arrive at a probable configuration of boundary conditions. Overpeck and Holden7 considered the effect of reservoir anisotropy on fault detection and showed that the distance calculated could differ by as much as 20% with those obtained by assuming isotropic medium. They gave a procedure for imaging when the principal permeability axes are at some angle other than 0 or 90° to the fault boundary. Rodgers and McArthur8 employed a minimum in standard deviation between observed pressures and a least-squares straight-line fit to determine boundary configuration. Prasad9presented a procedure to compute transient pressures for a well between two sealing faults intersecting at any angle. Jones10 in 1961 drew attention to the possible use of rate change of well pressure with time in detecting reservoir boundaries. Van Poollen11 presented graphs of time rate of change of well pressure during drawdown for various well locations between faults intersecting at 90 and 36°. Although this approach yields interpretable drawdown behavior, it has not been used widely in the petroleum literature. Witherspoon et al.12 considered the effect of a linear no-flow or flow boundary and presented the dimensionless pressure behavior caused by a constant-rate producing well at an observation well some distance away. They discussed techniques to analyze drawdown test data.