In much the same way as rock mechanics started to bloom in geotechnical engineering after catastrophes such as Malpasset and Vaiont, the petroleum industry started to respect rock mechanics after rather expensive mishaps. Boreholes failing during drilling or production, formation sand production, surface subsidence leading to casing failures, etc. all represent reactions of the rock mass to engineers' intrusion. As a result of these and other seemingly increasing problems, the exploration and development sectors felt obliged to address the issue and over the past two decades have expanded considerable efforts towards obtaining a better understanding of the fundamental mechanisms governing rock behavior. This keynote address briefly reviews some of the recent advances: Wherever possible, practical implications are discussed in an attempt to outline the importance of some of these developments.
The petroleum industry has been involved in rock mechanics ever since hydraulic fracturing treatments were performed in the mid forties for stimulation purposes. The original belief that manmade fractures were always horizontal had to be reexamined, whereupon the importance of the insitu stress field became obvious. It is only in the past fifteen years, however, that the oil and gas producers fully realized the importance of rock mechanics in many aspects of their industry. This 'renewed' interest was triggered by problems associated with the development of difficult reservoirs: lenticular, naturally-fractured, unand poorly-consolidated became the rule rather than the exception. In addition, these reservoirs often occurred in locations where the conditions were far from ideal: great depths, deep water, high pore pressures, unusual tectonic stress regimes, etc. Economic conditions, as well as fashionable trends, pressured the industry to consider highly inclined or even horizontal completions with all their associated problems.
The industry also quickly realized that traditional rock mechanics concepts developed for and by the mining industry had to be extended to include the important hydrocarbon phases. Consequently, coupled behavior had to be considered and poroelasticity, for example, became a hot research topic, leading to the development of new fundamental solutions. The pay zone depths were such that linear elastic solutions became rarely applicable; visco-elasticity, plasticity, and viscoplasticity needed to be considered in order to explain some field behavior. Fracture mechanics concepts were also called upon both on microscopic as well as macroscopic scales. By considering the rock to be pervaded by randomly-oriented microcracks, new fundamental failure mechanisms were unveiled. The nonlinear behavior of the fractures also shed light on production anomalies observed in some reservoirs.
These efforts have led to new insights which are being included in theoretical as well as numerical schemes that are and will be applied to practical field situations.
This paper will briefly review some of the most recent developments and will attempt to outline the major differences from the traditional approaches. It will follow the chronological order of a typical field development, from drilling to abandonment.