Abstract

Prehydraulic fracture diagnostic pumping analysis has recently improved with the use of new analysis techniques such as G-Function derivative plots, after-closure analysis, and step-rate tests. This paper analyzes various types and combinations of step-rate injection tests from many different formations around the world to determine the usefulness of these tests. The analysis uses wells with both surface and bottomhole gauge data, and in some instances, compares the results of the two. The final results of the stimulation treatments are also compared to the prefrac analysis. While the results of these tests provide information on the presence of excess near-wellbore friction or tortuosity, what is often not taken into account is that this tortuosity often destroys the usefulness of these step-rate tests in providing much sought-after data such as accurate fluid efficiency and closure pressure numbers.

The focus of this paper will be on step-up and step-down analysis, with the result being a new type of graph that provides an indepth look at the quality of these tests in any given well. Often these tests are performed and erroneously analyzed because of the effects of tortuosity, with the end result being either the data is ignored or discarded. Techniques are provided for analyzing these tests and suggestions are given to improve the results obtained from these tests.

Introduction

Oil and gas wells of different permeabilities and lithologies often need to be effectively fracture stimulated to provide operators with sufficient economic return on investment. In an effort to ensure that a stimulation treatment can be placed, injection tests or fracture stimulations without proppant or with minimal amounts of proppant have been employed to test a formation's capacity to receive a treatment and to help optimize the final treatment design. The design of these injection tests, usually called "minifracs" or "datafracs" is based on the type of information the operator or stimulation designer seeks. Information that can be obtained or inferred from these tests include closure stress or minimum stress, bounding stresses, fracture geometry, presence of natural fractures, permeability, leakoff coefficient, fluid efficiency, pore pressure, fracture gradient, fracture extension pressure, net pressure, and excess friction.[1–3] Variations that can be made in these tests include injection rate, fluid type, fluid loss additives, proppant type, proppant volumes and concentrations, and finally, combinations of various diagnostic injections. The order in which these tests are performed can also have an influence on the outcome of the analysis and final treatment design.

One such test is the "step-up" step-rate test. In this test, injection into a formation is begun at a slow rate for a fixed amount of time, and the rate is then increased and again held for the same amount of time. This is repeated in an attempt to achieve three matrix injection rates and three fracture injection rates. A graph of rate vs. bottomhole pressure is then made at the stabilized points, and fracture-extension pressure is indicated as the point where the pressure "breaks over" or large increases in rate provide small increases in bottomhole treating pressure. As will be discussed, a plot of bottomhole pressure vs. injection rate provides a myriad of useful information, provided there is good communication between the wellbore and the formation. It will also be shown that the presence of tortuosity virtually destroys this test, and while it has been proposed that near-wellbore friction can be mathematically removed from this test, the supplied analysis demonstrates that this is rarely the case.

Another rate-dependent test is the "step-down" step-rate test. It has been proposed and is now generally accepted that this test can provide a rate dependent friction value for tortuosity and perforation friction, and can differentiate between the two. The main requirements of this test are that it be sufficiently rapid, or sufficiently slow in the case of formations with very low leakoff, so that the fracture geometry does not change during the step-down test, and that a displacement fluid with known friction values or bottomhole pressure is accurately determined from a live annulus or bottomhole gauges.

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