A New Method To Improve The Accuracy Of Acoustic Pressure Calculations

Abstract

Abstract Acoustic pressure measurements can be an important surveillance tool in oil fields undergoing water flood and enhanced oil recovery schemes. They offer a cost-effective method for measuring subsurface pressures in pumping oil well without the expense of pulling the rods and pump. However, experience has shown that the accuracy of subsurface pressures obtained from fluid-level calculations has at times, proved questionable. Data from several well studies is presented that shows the average difference between standard acoustic pressure calculations and directly measured subsurface pressures is approximately 10%. Furthermore, a new method of calculating acoustic pressures is described. The new approach was applied to test data from nine producing oil wells in Alberta. All wells were chosen from either waterflood or miscible flood projects. For this particular set of well tests, the new procedure reduced the average difference between acoustic and directly measured pressures to within 1%. The new calculation method extends the applicability of the method described in the ERCB Guide G-5(1) to a wider range of oil compositions. The new method also addresses some of the concerns raised in the Guide G-5 and improves the accuracy of acoustic pressure calculations in water flood and miscible flood projects. Introduction In Alberta, the industry standard for the acquisition and interpretation of acoustic data is the ERCB Guide G-5 "Calculating Subsurface Pressure Via Fluid-Level Recorders ". It is an excellent technical reference that contains many specific and practical points about acoustic measurements on pumping oil wells. The Guide G-5 was developed in the early 1970s, and was based on comparative data collected from 1966 to 1968. The wells studied produced only oil and gas. No wells producing water were included. These assumptions were more than adequate for the time, and the Guide has proven to be useful for almost 20 years. The oil industry now faces different challenges in the 1990s. Waterfloods and enhanced oil recovery projects have become Commonplace. The "typical" oil well is no longer a simple gas and oil producer with easily predicted fluid gradients. Instead, it is a pumping well that produces gas, oil, water and in some cases, miscible solvent and chase gas. The Guide States, "Experience has shown that the accuracy of subsurface pressures obtained from fluid-level calculations has, at times, proved questionable. Although the producing characteristics of many wells preclude great accuracy, the lack of accuracy can be attributed in many cases to improper data-gathering procedures, interpretation, and pressure calculation techniques. " The Guide also discusses wells in water floods and states, "The producing water-cut is seldom representative of water within the fluid column in the annulus, and should not be used when making calculations." The Guide indicates instead, that the water cut in the annular column will decrease over time. Despite these guidelines and statements, it has become standard industry practice to assume that the incoming liquid water cut is constant and equal to the final producing water cut. Acoustic Verification Surveys(2,3,4) In order to investigate the accuracy of the acoustic pressure calculations, verification surveys were conducted on nine wells operated by Amoco Canada.