This review introduces various items offered under the common subject heading "Influences of fluid pressure changes on (sedimentary) rock deformation, yield and fracturing". Three examples related to major problem areas in oil and gas production practise are discussed in terms of simplified mechanical models. Together they cover a variety of rocks mechanics applications; hydraulically induced fracture propagation; the effect of "granular" rock creep on reservoir compaction leading to subsidence; and prediction of sand-influx due to production, based on Coulomb-Mohr plasticity. Although the techniques used are primarily classical, a survey of the state of the art reveals that additional practical information can still be extracted from such basic considerations.
The availability of large mainframe computers to research is a mixed blessing. On the one hand it opens entirely new avenues. A striking example is the emergence of fractal mechanics, that may for instance improve our understanding of oil displacement by water or gas in porous media. On the other hand it can lead to over complicated treatment of some engineering problems if the necessary input know how is inferior to the computational abilities of the program. Rock mechanics of oil and gas reservoirs deals with materials that exhibit heterogeneous elastic, creep, plastic and strength properties. Borehole logs and core analysis reveal some material properties, but only at specific locations, like needles in a haystack. Simplified analytical treatment of natural or imposed processes remains of paramount importance to the petroleum engineer. The task of the engineer is to estimate by appropriate modelling the consequences or effectiveness of attempts to produce oil or gas in the most economic manner. For certain engineering problems average material property values are sufficient. Other problems require detailed spatial and temporal distributions of properties, often not available in petroleum problems. In this general report three examples are presented. They are based on recent reported progress in simplified mechanical modelling of elastic, viscous and plastic rock deformation.
- –
Linear elastic case: Prediction of the dimensions of hydraulically induced factures,
- –
rock creep: Prediction of sandstone reservoir compaction leading to surface subsidence, and,
- –
elastic - plastic analysis: Prediction of the risk of sand influx during production.
The past decade has seen the introduction of fully 3-dimensional hydraulic fracturing simulators. They appear to be research rather than engineering or design tools, because,
- –
the necessary mechanical and loading conditions away from the wellbore are frequently a matter of guesswork,
- –
computer-run time, even for fast mainframe computers, can be considerable, and,
- –
the results cannot easily be included into computational schemes aimed at treatment optimization. Therefore, so-called fixed height, 2-dimensional models, are still used in optimization schemes. For example, Perkins/Kern/Nordgren (PKN)¹ and Geertsma/ de Klerk2 (GdK) developed analytical formulae for fracture width, length and wellbore pressure, including the effect of fluid-loss. To obtain in-situ information on fluid-loss parameters and mechanical properties, minifrac treatments are carriedout.
