Interpretation of Underbalanced Surge Pressure Data by Rate-Time Convolution

Abstract

ABSTRACT Well test analysis of pressure response data from either underbalanced perforating or backsurge perforation washing is accomplished by rate-time convolution. The presented method allows closed chamber test analysis of reservoir transmissivity and wellbore skin by a straight line graph analogous to a Horner plot. As a well test analysis method, the closed chamber test has the benefit of avoiding surface pressure buildup, which often prohibits conventional drill stem testing. Pressure measurement is required at the top of the surge air chamber and at the sand face. A linear plot is generated by convolution with respect to time of flowrate, which is derived from the air chamber pressure, and plotting the bottom hole pressure versus the rate-time superposition function. Transmissivity is obtained from the slope of the pressure data during the period when afterflow is minimal and pressure is not sensitive to skin. An estimate of initial reservoir pressure results from extrapolation of the pressure trend. Subsequent forced fitting of the flow period data to the expected linear plot of bottom-hole pressure versus the rate-time superpostion function yields wellbore skin. Unique to the convolution method, wellbore skin may be calculated for each flowing pressure measurement. Therefore In addition to final skin determination, the effectiveness of the perforation cleaning mechanism may be observed by noting the trend of skin as a function of time during the flow period. Example backsurge analyses are presented from the offshore Gulf of Mexico. Transmissivity values are calculated from the slope of the late time pressure data. The examples illustrate how wellbore skin varies with respect to time during the perforation cleaning flow period. One example illustrates effect of reservoir boundaries on the backsurge pressure response.