Influence of Permeability Anisotropy and Reservoir Heterogeneity on Optimization of Infill Drilling in Naturally Fractured Tight-Gas Mesaverde Sandstone Reservoirs, San Juan Basin

I t is essential to determine fracture orientation w i t h i n a reservoir i n order to explore and develop fractured reservoirs properly. Where surface exposures are poor, determination of fracture orientation has previously been available only by acquiring and studying oriented cores. A new method of analysis and presentation has recently been used successfully to determine orientation f rom wireline dipmeter-type fracture logs f r o m the Gavilan Mancos oi l pool i n the San Juan basin of New Mexico. Wireline dipmeter-type fracture logs are most often used wi th in the wellbore to detect fracture occurrence in the formations around the wellbore. When data f rom these logs are plotted i n a rose diagram presentation, a generalized orientation of the fracture sets around an individual wellbore can be determined. Used i n combination w i t h a structure map of the area, these orientation data can aid in the intepretation of fractures w i t h i n the reservoir. I N T R O D U C T I O N The study of fractured reservoirs and associated fracture systems within conventional oil and gas reservoirs has gained momentum in recent years. Two investigators, Aguilera 1980) and Nelson (1985) have published excellent books which combine their own experiences with complete bibliographies on the subject of the formation and study of naturally fractured reservoirs. Harris et al (1960) recognized that frequency and orientation of fracture sets are dependent on a combination of both regional and local structural configuration. Where surface mapping is impractical, fracture azimuth can be obtained from oriented cores. It has been shown that there is good correlation between fracture orientation data derived from oriented cores and surface mapping (John F. Harris, 1987, personal communication; Quillin and Stearns, 1986). Fracture sets can be identified in cuttings (Harris, 1979) but the orientation of the angles cannot be determined. The presence of fractures in the subsurface can also be inferred by analyzing one of the many types of wireline logs or by utilizing other special techniques available. There is little information available however, about the use of wireline logs to determine fracture orientation. Until recently, in depth analysis of fracture orientation for exploration, development and reservoir studies could only be accomplished by conducting detailed surface mapping projects or by obtaining and analyzing cosdy oriented core. A new method has been developed to utilize existing data •d acquire additional low cost data to understand the jcurrence and orientation of fracture patterns. During the course of a detailed study of the Gavilan Mancos oil pool, Rio Arriba County, New Mexico, it became evident that the dipmeter-type fracture log could also be used to understand the structural relationship of the fracture patterns within the reservoir. This fracture log was run in addition to the standard log suite by many companies in the latter portion of the development of this field. To develop this new method for determination of fracture orientation within the reservoir, all available dipmeter fracture logs were examined. All the observed fractures and their orientations were systematically identified and the resulting data displayed in a meaningful fashion. METHOD OF INVESTIGATION Accumulation of orientation data starts with the determination of the fractured zone of interest. This can be acomplished by a comparison of drill cuttings analysis, wireline log examination and production data. Once the fractured zones are determined, a consistent interval of study is selected. This should include at least 100 feet (30.76 m) above and below the major fractured interval to help identify any zones of caving. Within the study interval, detail analysis of the dipmeter fracture log is undertaken. Each fracture is identified and assigned a number. This number is recorded on a datasheet (Fig. 1) along with the depth, notation of the pads reading the fracture, and the azimuth of pad #1, which is always oriented on the log. From the data, the strike of each fracture can be calculated from polar plots adjusted for the magnetic declination of the area. Each pass of the zone by the logging tool should be analyzed. Data from only one pass should be held in suspect, with a minimum of two passes deemed adequate for proper rho Mnuntain Genlnaist Vol 26, No. 2 (ADfil. 19B9). p 63-67. The Rocky Mountain Association of Geologists 63