A number of developments over the past five years have led to dramatic changes in the design and execution of hydraulic fracturing treatments. Five primary new issues are described: convection, tortuosity, rock/fracture(non-linear) response, appropriate use of rheology/flow-rate and permeability variation effects. Consequences include "paradigm shifts", such as inverse relations between appropriate proppant concentration and reservoir permeability. Resulting integrated cost-effective and rewarding methodologies for fracture execution are emphasized and the methodology is illustrated with just a few case-studies, selected to emphasize a variety of technical features as against absolutely optimum execution. Suitable generic implementation and procedures, using greatly reduced fluid volumes, are outlined in sufficiently general form to allow (practically optimum) job execution on any well, particularly in low-permeability reservoirs.
The variation between (nominally identical) wells, and the vital role of real-time analysis for on-site execution of adequate jobs, is demonstrated -including unique (but essential) representation of realistic/general physics in the models used to interpret adequate data-sets. The case-studies serve to illustrate the major issues, including many ancillary features involved in proper interpretation of pressure/flow data before, during and after fracturing treatments: it is emphasized here (and in a companion paper for higher permeability) that the proper/sensible combination of all freely-collectible data, including long-term production plots, which should be a natural part of all well operations, usually leads to (practically) unique interpretation of reservoir conditions and job effectiveness: good reservoirs are properly identified and poor success cannot be so easily ascribed to poor kH. Well procedures and costs are thereby adapted to (optimum) production.
Introduction
Hydraulic fracturing may be considered by many to be a mature technology, certainly when viewed in the light of old monographs (e.g. Ref. 1) and more recent treatises (e.g. Refs.2, 3) which serve to represent established concepts/procedures. Unfortunately, most of the associated concepts and methodologies have been developed in the absence of much worthwhile data acquisition and analysis, especially in regard to field operations: such very limited data, when available at all, has allowed excessively free reign over interpretation and mandating of associated procedures. The result, common to most activities with limited data control, has been a proliferation of arbitrary justifications for individual preferences; in the best cases, limited field experiences/observations have been misinterpreted and readily perceived problems (such as early screen-outs) have been remedied only at the expense of creating much greater problems (e.g. poor proppant placement).
Experiences over the past ten years (e.g. Refs. 4-9), including much testing of major procedural modification over the past five years (e.g. Refs. 10-15), have led to major revisions of industry thinking and a whole new approach to hydraulic fracturing. Such efforts have involved many controversial departures from long-held beliefs (including some of those which evolved from prior work, e.g. Refs. 16-20): the resulting dramatic impact, especially when opportunities finally arose to go "all the way", involved many procedures opposite to what had become accepted practice by most operators in the industry. This rapid evolution/execution has been made possible and supported by parallel development of a unique capability for real-time (real-data) analysis, based on a PC- computerized system, which can now free the operating engineer from the shackles of institutionalized thinking (e.g. limited to "type-curves"), leading to practically effective/optimum jobs.
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