Dynamic Etching Tests Aid Fracture Acidizing Treatment Design

Abstract

Abstract Satisfactory stimulation of oil and gas bearing formations by "fracture acidizing" is dependent upon development of adequate fracture conductivity in naturally occurring or hydraulically induced fractures. The created fracture conductivity is a result of the action of acid on the exposed fracture face. Some of the kinetic parameters which will influence the resultant conductivity are acid type and strength, reaction temperature, reaction time, and flow regime. These parameters are treatment design variables which can be used to optimize fracture acidizing treatments. Formation characteristics will be equally important, and in many cases, more critical than the kinetic parameters. Some of the formation parameters which will affect the development of fracture conductivity are the mineralogical composition, degree of homogeneity, hardness, and closure stress. Equipment has been developed to allow realistic laboratory simulation of dynamic etching during the fracture acidizing process. Dynamic etching tests can be used to determine the feasibility of an acid treatment, as well as allowing optimization of the treatment parameters discussed above. Equipment construction allows evaluation of all strengths and forms of acids —e.g. non-gelled, gelled, foamed, emulsified, or chemically retarded — to be evaluated at temperatures up to 375°F, and at closure stress levels up to 12,000 psi. Any type of formation core may be evaluated. Recent improvements in dynamic etching tests have allowed successful stimulation of difficult to treat formations such as cold dolomites, chalks, formations with low acid solubilities, and uniform carbonates. Introduction The use of various fracturing methods for stimulation of wells has become a common procedure in the oil and gas industry. Fracturing treatments are performed on wells of various potentials to help increase production. Concurrent with the desire for increased production is the need to optimize treatment designs and predict what increase might be expected. This is true regardless of whether the stimulation method is hydraulic fracturing with proppants or fracture acidizing. Over the past 25 years, numerous authors have investigated the factors which will affect the production increase of a fractured well. Early studies utilized curves developed from electrical and mathematical models to predict theoretical production increases. The advent of high speed computers has allowed production increase calculations to advance to the point where the theories of transient pressure analysis are utilized in conjunction with numerical simulators or type curve simulators. Regardless of the method selected, all production increase calculations indicate that three factors are critical to the success of a fracturing treatment: effective fracture length, fracture height, and fracture conductivity. In the field of hydraulic fracturing with proppants, the ability to predict fracture geometry, hence fracture length and height, has always been an important research topic. B- 11 Recently, there has been a dramatic increase in information on the subject of realistic downhole propped fracture conductivities.