Preformed Particle Gel for Conformance Control: Factors Affecting its Properties and Applications

Abstract

Abstract Injecting preformed particle gel (PG) as a fluid-diverting agent to reduce water production is an attractive new procedure designed to minimize some of the risks inherent in gel treatments based on in-situ gelling. The objectives of this paper are:understand how to control PG properties by changing gelant compositions and their fraction when synthesized;determine where PG can be applied and how reservoir conditions affect PG properties; andoutline candidate well criteria and the proper injection procedures by illustrating several field applications. Based on laboratory experiments, the following results will show that:PG strength and swelling capacity can be controlled by adjusting gelant compositions;certain additives can improve PG stability at elevated temperatures (120°C);increasing temperature will increase the swelling ratio and the swelling rate of PG, and increasing salinity will reduce the swelling capacity of PG and will increase PG strength;the swelling capacity of PG is insensitive to pH;PG is insoluble in water, but absorbs it, swelling up to 20–200 times of its original size. It is strength- and size-controlled, environment-friendly and not sensitive to reservoir minerals and formation water salinity. Three examples from more than 200 operations were selected to show how to choose candidate wells and how to operate the injection procedures. PG can be used as a conformance control agent to correct permeability heterogeneity for those reservoirs with fractures or channels, both of which are widely found in mature water-flooding oilfields in China. Introduction Most oil fields in China were found in continental sedimentary basins. They are characterized by complex geologic conditions and high permeability contrasts inside reservoirs. To maintain or increase the driving force, these oilfields were developed by water injection. However, serious vertical and lateral heterogeneity of formations resulted in rapid water cut increase of production wells. Moreover, sand production and dissolution of rock matrix further worsen reservoir heterogeneity. Severe heterogeneity of reservoir has become one of the most important problems that reservoir engineers are facing. Many inter-well tracer tests have shown that channels or fractures widely exist in most oilfields, no matter whether they have fracture or not at their early development stages. In this paper, "channel" means an open, linear-flow structure. It does not mean flow through matrix. Another example of worsen heterogeneity comes from the application of clay gel in China Oilfields. Many injection wells have been successfully injected hundreds or thousands tons of clay respectively. If we calculate the permeability from Darcy law according to the "1/9~1/3 rules", [1–2] which is a rule about how to match pore throat and penetration particle size, the formation permeability should be more than one thousand Darcy [3]. Moreover, earlier polymer breakthrough in Daqing, Nanyan, and Shenli oilfields further proved channels or fractures are widely spreading in most oilfields. [3] To control water cut and improve oil recovery of oilfields, many technologies, such as polymer flooding, surfactant flooding, foam flooding and so on, have been taken in China so far. [4–7] One of the most popular methods is to inject gel to reduce flow capacity of channels or fractures and divert following fluid (normally, water) to unswept oil zones. [8,9] Before 1990's, gel treatment concentrated on correcting permeability heterogeneity near wellbore (normally 5~10meters). But in-depth gel treatment technology has become more and more important for oilfield owners because heterogeneity of thick net pay has become main problem influencing oil recovery of mature oilfields and gel has been injected multi-times in some wells respectively so that no remaining oil is left near wellbore. [10]