Fracturing treatments using treated water and very low proppant concentrations ("waterfracs") have proven to be surprisingly successful in the East Texas Cotton Valley sand. This paper presents field and production data from such treatments and compares them to conventional frac jobs. We also propose possible explanations for why this process works.

Waterflooding of California's diatomite reservoirs has heen extensively employed for two reasons: (1) to increase total recovery, and (2) to mitigate the potentially catastrophic effects of reservoir compaction and the resulting surface subsidence. Waterflooding has typically striven to replace each barrel of produced fluid with a barrel of injected water in order to achieve "zero net voidage." The extremely low permeability of the diatomite reservoirs, however, results in the generation of very significant reservoir pressure gradients during waterflooding, even under zero net voidage conditions. These extreme gradients in reservoir pressure, together with the reservoir compaction, result in significant changes in the local reservoir stress field. These local stress perturbations can, in turn, result in reorientation of hydraulic fractures on infill wells and possibly contribute significantly to the potential for wellbore casing failure.

This paper summarizes the (preliminary) findings from extensive field studies of hydraulic fracture orientation in diatomite waterfloods and related efforts to monitor the induced surface subsidence. Included are case studies from the Belridge and Lost Hills diatomite reservoirs. The primary purpose of the paper is to document a large volume of tiltmeter hydraulic fracture orientation data that demonstrates waterflood-induced fracture reorientation-a phenomenon not previously considered in waterflood development planning. Also included is a brief overview of three possible mechanisms for the observed waterflood fracture reorientation. A discussion section details efforts to isolate the operative mechanism(s) from the most extensive case study, as well as suggesting a possible strategy for detecting and possibly mitigating some of the adverse effects of production/injection induced reservoir stress changes - reservoir compaction and surface subsidence as well as fracture reorientation.

Hydraulic fracture orientation is critical to both primary and secondary oil recovery in low-permeability reservoirs. Incomplete and often overlapping drainage patterns under primary recovery, as well as inefficient sweep and premature water (or steam) breakthrough under secondary recovery are some of the common production problems that often result from hydraulic fracture reorientation. Often, hydraulic fracture orientation is measured on a few wells, and then generalized across the entire field under development. This characterization of regional fracture (stress) orientation is then assumed constant over the development life of the field. A wealth of recent observations have definitively shown that fracture (stress) orientation in low-permeability reservoirs can be profoundly affected by production activities.

Hydraulic fracture reorientation has been observed on dozens of staged fracture treatments (in several fields) under both primary and secondary recovery. A summary of collected field data from three extensive field studies is presented. The production impact of fracture reorientation on both primary and secondary recovery schemes is addressed; and strategies are presented which utilize the recent findings for both enhancing primary recovery and mitigating some common problems with secondary recovery.

The discussion of reorientation mechanisms is greatly enlightened by recent data which reveals a startling correlation between observed fracture reorientation and indirect measurements of reservoir compaction.