Perforating and Hydraulic Proppant Fracturing in Western Siberia, Russia

Abstract

Abstract Hydraulic proppant fracturing is developing rapidly in the oil fields in western Siberia and is seen as the most important means to improve oil production from both existing and newly drilled wells. The oil fields vary in size and reservoir quality, with a majority of the developed fields showing permeabilities of 1 to 30 mD. Reservoirs are layered, and fracturing vertical wells in general seems to give higher productivity compared to horizontal wells that are not being hydraulically fractured. This result is most likely because the vertical permeability is much lower than the horizontal permeability. To maximize production from these wells, coarse-sized proppant must be placed using the least-damaging and lowest-viscosity fluid systems. For production optimization, the trend is currently "larger (proppant sizes) and bigger (jobs)," using the lowest acceptable polymer loading in the frac fluids, which minimizes gel damage. This paper describes the conflicts that can arise between the requirement to place large amounts of coarse propping material using low viscosity fluids and the perforation size and condition that is needed to do so. These conflicts can lead to job scenarios in which screenouts take place. During fracturing operations in wells in west Siberia, some screenouts did take place. An investigation of the screenouts revealed that the major cause of the screenouts is reduced width development at the perforations associated with the "stress cage." Guidelines were developed to reduce the influence of the near-wellbore stress cage and prevent screenouts. In addition to these guidelines, future solutions are proposed that may avoid the stress cage and place coarse proppant more efficiently. Introduction The objective of perforating is to provide a conductive flow path from the formation, through the cement sheath and casing, and into the completion tubulars. In a fracture-stimulated reservoir, the perforations connect the fracture with the wellbore. The many perforating options include tubing-conveyed or wireline-conveyed systems, casing or through-tubing guns, and deep-penetrating (DP) and big-hole (BH) charges. Furthermore, several options are available for gun phasing and shot density, and perforating can be performed overbalanced, balanced, or underbalanced. The specific option selected has a significant effect on the success of a stimulation treatment, and particularly so in propped hydraulic fracturing. Perforating charges are tested in a surface apparatus under controlled conditions. The charge, gun system, test conditions, and test results are recorded according to specifications APIRP19B and previously API-RP43. Because the compressive strengths of the targets used in testing can vary greatly, care should be exercised in using these specifications to compare charge/gun performance between vendors. Furthermore, because of the following reasons, simple extrapolations of the test results to downhole performance are usually not possible:The surface tests do not account for the effects of net effective stresses (overburden gradient effects and pore-pressure effects).The entry-hole data are obtained in low- to moderate-strength casing (J-55 to N-80) and the hole size decreases in high-strength casing, such as P-110. Computer programs have been developed that use the API surface data to predict downhole performance. Table 1 shows an example of the performance data at surface and at downhole conditions for a typical perforating gun arrangement used in western Siberia. The downhole performance data are shown in Fig. 1. Perforating and Hydraulic Fracturing Several papers have been written on the importance of perforating for stimulation and hydraulic fracturing in particular, both from a technical as well as economical standpoint.1,2 In addition, several papers have been presented on fracture initiation, multiple fractures, near-wellbore-related screenouts, and other related subjects.2–12