Pressure Pulsing - An Improved Method of Waterflooding Fractured Reservoirs

Abstract

Abstract The basic idea of oil recovery by waterflood pressure pulses is discussed and supported by field evidence including accidental pulsing of a Permian Basin, Grayburg reservoir. Laboratory tests on "fracture matrix blocks" which represent a portion of a fractured or vugular oil reservoir are described. Field and laboratory data show that cyclic pressuring/ depressuring by water injection may have considerable merit over conventional water flooding in applicable reservoirs. The data also show that supplementary gas injection can significantly improve the process. Introduction Fracture networks or interconnected vugs or solution channels are generally a bane to conventional water flooding because they cause fluid water channeling and oil by-passing. An improved secondary recovery process for these types of reservoirs is discussed in this paper. This process, called "pressure pulsing" by Owens and Archer, differs from water flooding in that it is specifically designed to take advantage of these networks and to use them alternately as avenues of injection and production. In pressure pulsing oil recovery occurs largely by imbibition of water from the fracture network into the fine pores of the fracture blocks followed by countercurrent flow of oil from the fine pore structure into the fracture. This is analogous to the action of a blotter when it imbibes liquids and expells air from its internal pore space. The paper discusses a field example in a Grayburg reservoir which supports the concept of pulsing. The basic imbibition process has been the subject of intensive research by many oil companies since 1952; its initial field application, however, in the Spraberry field was disappointing from an economic point of view. Pressure pulsing overcomes the shortcomings of imbibition flooding in two ways:injection is speeded up from, the slow imbibition rates to capacity rates, andinjection is interrupted to facilitate flow of oil from the rock matrix to the fracture network and thence to the producing wells. From laboratory evidence and the field data available, we expect that pressure pulsing could have several advantages in applicable reservoirs over conventional water flooding. In addition this work shows that the process can be improved by more efficiently using the gas energy remaining in the reservoir. We have also demonstrated that supplementing the gas energy by gas injection together with water injection has considerable merit. HOW PRESSURE PULSING BEGAN Atlantic's researchers studied oil recovery by imbibition on Spraberry cores in 1952 and then carried the process to the field in 1952 through 1955. Based on laboratory results, field recoveries of about 30 percent of oil initially in place were hopefully anticipated. Results of the field test, however, were disappointing and showed that imbibition alone was too slow a process to be of practical value. The first pilot project by Atlantic was followed by other pilot water injection projects by Humble in 1955 through 1958 and by Sohio starting in 1961. These latter projects gave rise to important modifications to the initial imbibition flooding concept. Thus, water injection was speeded up from slow imbibition rates to capacity rates.