Analysis Of Mechanisms And Procedures For Producing Favourable Shapes Of Hydraulic Fractures

Abstract

ANALYSIS OF MECHANISMS AND PROCEDURES FOR PRODUCING FAVOURABLE SHAPES OF HYDRAULIC PRODUCING FAVOURABLE SHAPES OF HYDRAULIC FRACTURES Abstract The paper identifies some primary features of tetonic conditions, rock response and fluid rheology which can strongly affect the shape evolution of hydraulic fractures in representative reservoir formations. Numerous mechanisms are described, with emphasis on the question of fracture containment within regions of desirable transmissivity increase. Relevent natural characteristics include: contrasts of in situ stress, variations in material deformability/fracturability and differences in permeability between pay-zones and adjacent strata. Various degrees of crack-arrest are possible but the dominant component of most mechanisms is the retardation of fracture growth caused by impedance to fluid flow. Artificial biasing of flow patterns may be achieved, by chemical or mechanical patterns may be achieved, by chemical or mechanical preparation of the region to be stimulated and by use preparation of the region to be stimulated and by use of complex frac-fluid rheology. Implementation of these phenomena in practical designs typically requires detailed analysis with numerical models, but numerous recommendations can be made to improve conventional practises: for instance, fracturing pressures must be practises: for instance, fracturing pressures must be carefully limited, pretreatment can be helpful and rheology/proppant-scheduling can be exploited to much greater extents. INTRODUCTION AND OUTLINE The problem of producing underground fractures with suitable extents and shapes has taken on a very great importance in the last few years, due to well-appreciated portentions of shortages in oil, gas and other natural resources. This has led to much research and development activity on the many varied aspects of both potential and existing reservoir access techniques. One such topic has been the production of suitably contained hydraulic fractures, this approach is based on a well-established technology and, of all the methods that have been proposed, it seems to promise the most fruitful basis for general application. promise the most fruitful basis for general application. Thus, we concentrate on this subject in the present paper, although the mechanisms and procedures described paper, although the mechanisms and procedures described have much broader implications for the growth of cracks in geological structures of all kinds. The main object of the paper is to describe some of the tectonic, structural, rheological and other physico-mechanical factors which play an important physico-mechanical factors which play an important role in deciding how an induced fracture will spread underground. As well, we suggest a number of modifications or rationalizations of current stimulation technology which could lead to improvements in the outcome of increasingly expensive treatments. A central theme is that the hydrofrac process is dominated by the requirement of fluid flow and all controlling mechanisms (except perhaps crack arrest) have this as their primary ingredient; only rarely do the simpler static crack calculations, currently popular in the literature, have much relevance for computing the shapes of fracture evolution. However, there are so many aspects, which govern the fracturing process in any particular formation. Each reservoir should be analysed in great detail on an individual basis; models are currently being developed to allow such realistic simulation. The mechanisms described here dictate some of the primary features which must be present in these models if they are to produce an optimum (or even credible) design for a fracture stimulation job. The paper is divided into eight sections; the first seven discuss mechanisms dictating the evolution of fracture shape, while the last section describes some of the simpler methods for improving the performance in some typical reservoir structures. The following categorization is adopted in describing mechanisms, although these are obviously interrelated:Mechanisms related to contrast in confining stresses acting on the various strata or segments of the reservoir.