Determining the 3-D F’racture Structure in the Geysers Geothermal Reservoir

In t roduc t ion The bulk of the steam a t the Geysers geothermal field is produced from fractures in a. relatively impermeable graywacke massif which has lieen heated by an underlying felsite intrusion. The largest of these fractures are steeply dipping right lateral strike-slip faults which are subparallel to the N W striking Clollayomi and Mercuryville faults which form t,lie NE and SW boundaries of the known reservoir. \\:lierc the graywacke source rock outcrops at the surface it is highly sheared and fractured over a. wide range of scale lengths. Boreholes drilled into the reservoir rock encounter distinct “stea.ni entries” a.t \vliicli t,he well head pressure jumps from a few to more t.lian one hundred psi. This observation that stean.is produced from a relatively small number of major fractures has persuaded some analysts to use t,he U’arren and Root (1963) dual porosity model for rescrvoir simulation purposes. The largest fractures i n this model are arranged in a regular 3-D array which partitions the reservoir into cubic “mat,rix” blocks. The net storage and transport contribution of all t.lie smaller fractures in the reservoir are lumped into average values for the porosity and perniealiility of these matrix blocks which then feed the large fractures. Recent improvements of this model largely focus on a more accurate representation of the t.ra.nsport from matrix to fractures (e.g. Pruess et al., 1983; Ziminerman et al., 1992), but t.he basic geoinetry is rarely questioned. However, it has long been recognized that steam entries often occur in clusters separated by large intervals of unproductive rock (Thomas et al., 1981). Such clustering of fixtures at all scale lengths is one characterist,ic of self-similar distributions in which the fracture distribution is scale-independent. Recent studies of the geometry of fracture networks both in the laboratory and in tlie field are finding that such patterns are selCsiniilar and can be best described using fractal geometry. Theoretical simulations of fracture developmeii t in heterogeneous media also produce fractal pa,tterns. However, a physical interpretation of the mechanics which produce the observed fractal geometxy rcmains an active area of current research. Two hypotheses for the physical cause of selc-similarity are the Laplacian growth of fractures in a self-organized critic,al stress field, and the evolution of percolation clusters in a random medium. Each predicts a different, fractal dimension. The inore important questions from a reservoir engineering point of view are: 1) is tlie network of fractures in the Geysers reservoir fractal and if so over wha.t range of fracture sizes i s tlie self-similarity observed and what is its fractal dinic.iisioii. a n d 2) do the conventional dual porosity numerical simulation schemes provide an adequate tlescript.iou of flow and heat mining at the Geysers? 0t.lier papers in this volume by Acuna, Ershaghi, and Yortsos (1992) and Mukhopodhyoy and Sahimi tlie second queslion. The primary ohjectivc of this pa.per is to try to answer the first. Toward this goal we have mapped fracture patterns in surface exposures of the graywacke source rock at the outcrop scale (meters), a t the road-cut scale (tens of meters) a.ncl a.t the regional scale (kilometers). We have also examined cores collected at depth from the graywacke reservoir rocks, and analyzed drilling logs making use of the pattern of steam entries as well as the fluctua.tioiis in drilling rate. Mapping f r ac tu re patterns at The Geysers field The graywacke reservoir source rock outcrops at several locations in the Geysers geothermal field. One particularly good location is at well site GDC-21 where the drilling pad has been cut from the hillside revealing a near vertical wall of graywacke which is about 100 feet long and 40 feet high. The graywacke a.t this location has a cataclastic texture where the largest clasts are on the order of 10 feet in diameter. Although the outcrop appears to have a fluction structure suggesting a component of ductile deformation, close inspection of the folds shows that they a.re composed of multiply fractured graywacke layers in which virtually all the strain appears to have been accommodated by brittle fracture. The largest clasts contain complex fracture patterns which are easily visible due to the infilling with minerals of a contrasting color. In fact, a t this outcrop virtually all of tlie fractures have been filled. The fracture patterns in three such clasts were mapped from photo mosaics and one exa.mple is shown as Fig 1. A sec-