Experimental and Modeling Evidence for Major Changes in Hydraulic Fracturing Design and Field Procedures

Abstract

The paper presents some important conclusions obtained by analysis of data from many hydraulic fracturing treatments monitored over the past five years. These conclusions involve some major changes from conventional concepts about hydraulic fracturing, many of which have been based on inadequate models of the process and a commensurate lack of adequate data or motivation to check those models. Recommendations are, therefore, also made about simple, low-cost procedures for adequate data collection, such as the use of flow-rate changes and/or multiple injection/shut-in cycles for stringent model evaluation. Our conclusions from such careful analysis include: shorter, wider fractures; relative insensitivity of fracture width to frac-fluid rheology; dangerously fast convection vs. settlement of proppant in imperfectly-contained fractures; and a potential role of natural fractures in explaining many phenomena formerly regarded as evidence for long contained fractures. Although serendipity may sometimes compensate for poor job design, we recommend that many existing approaches to fracture design and execution be re-considered and that more credible efforts at treatment optimization be achieved by more careful (on-site) analysis of properly monitored and more flexible field execution schedules. lIItroductionThe theory and practice of hydraulic fracturing abound ",":ith concepts, analysis techniques and procedures for creat~g "optimum fracture geometries" in representative reservoIrS.References and illustrations at end of paper. 131These ideas have evolved somewhat over the past twenty years, as may be seen, for instance, by comparing Ref. I and Refs. 2, 3: the most obvious, but still hard-won, recognition has been that fractures are typically not well-contained in the pay-zones of the target formations. However, many other misconceptions and inadequate models or procedures still pervade the industry -despite large amounts of data which, directly or indirectly, refute those approaches. This situation is still possible because adequate data-sets are not often collected and are rarely analyzed properly, if at all; and unsatisfactory explanations are often found when anomalies are noted (e.g. Ref. 4). Indeed, clearly questionable models are still purveyed (e.g. Refs. 2,3) as adequate representations of the physical process: in particular, they do not often come close to matching the actual observed pressures, even in well-defmed job situations. Results vary from vague (e.g. Ref. 5) to demonstrably erroneous recommendations for optimum job design. One of the authors (Geary) has been working on this problem for over twelve years, fIrSt conducting laboratory (Ref. 6) and computer (Ref. 7) simulations, then applying the resulting practical computer-based models (Refs. 8,9) to field applications, using actual data and on-site analysis (e.g. Ref. 10). Some of our original concepts (e.g. the role of rheology) have changed quite dramatically (e.g. from those presented in Ref. 11), but others (such as the roles of stress, modulus an...

Proppant Convection and Encapsulation in Hydraulic Fracturing; Practical Implications of Computer and Laboratory Simulations

“…13) that the (true net) pressure recorded is relatively insensitive to fluid type (e.g. slickwater vs. linear gels vs. cross-linked gels).…”

“…25, which (mechanistically) may be treated as a special case of the general approach presented in Ref. 13. The observed net pressures are much greater than those calculated by the elastic single-fracture models [e.g.…”

“…Refs. [13][14][15][16][17] to provide the capability for essential on-site (re-) design and realistic optimization of job execution. We have incorporated an approximate description of convection in this system, which is still (often) too conservative in its estimates of the degree to which convection dominates the proppant placement: to counter this short-coming, we have found that we must be extremely aggressive in job execution (see section on Practical Implications).…”

“…like SFE3 in Ref. 13) and the job is designed first with a large pad ( Figure 5), then (aggressively) to achieve pad depletion right at the end of the treatment (Figure 6). Both jobs result in reasonably effective proppant placement opposite the pay zone ( Figures 5(a), 6(a)) and the pressure responses are not greatly different ( Figures 5@), 6@)).…”

“…Figure 1) for resolution of problems, without undue expense, in routine commercial operations [Refs. [13][14][15][16][17].…”