Gohfer 测试压裂分析理论和实践

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There are various ways to analyze step-down data to determine perforation and near-well friction. The step-down data are preferred since the fracture is created and fully inflated and the dynamic pressure response to rate changes should be related almost completely to frictional components. Any step-down analysis conducted on surface pressure data first requires an accurate pipe friction estimate. Once this is accounted for, any additional ratedependent pressure drop is caused by perforation and near-well flow restrictions. Perf pressure drop is related to the square of rate while tortuosity is generally related to the square-root of rate. This allows the two effects to separated when multiple rate data are available. The model above shows a discrete match of each frictional component over the entire step-rate test.
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The conventional step-rate test analysis plots observed pressure versus pump rate. A break in the curve indicates fracture extension pressure. The slope of the curve after breakdown indicates overall friction in the fracture and wellbore. In this case the data are plotted for both increasing and decreasing rates. The hysteresis in the curve illustrates changes in net pressure and fracture extension conditions that occurred during pumping.
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Pre-frac testing is conducted to provide specific diagnostic information about the reservoir, wellbore, fluid, and completion geometry. Various data can be obtained from different types of tests. The testing procedure must be designed to provide the maximum amount of critical data for the least cost, in terms of induced fluid damage and perturbation of the reservoir system. Every injection carries the risk of inducing some damage or alteration in the native stress and saturation state. The more complex and potentially damaging the fluid system becomes, the more the chance to overwhelm the data that is being extracted increases. In the past pre-frac testing has been directed toward characterizing the frac fluid itself. The philosophy recommended here attempts to characterize the reservoir and completion, while sacrificing information about the fluid.
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The plot shows actual data from an extended injection/falloff test. This test was somewhat longer than usual but shows the main features of the step-rate test and falloff.
Pre-frac injection diagnostic tests are an important source of information to characterize reservoir properties and in-situ stress state. These tests can provide direct measurements of many parameters that are critical to the design and evaluation pro the individual step-rate pressure data points are plotted as a function of rate on a Cartesian plot. The wellbore geometry and expected perforation diameter and entry coefficient are specified. In the figure the fixed input values are highlighted in yellow. The frictional character of the fluid is also specified through n’ and k’. The number of perforations open and the tortuosity pre-factor, shown in blue highlights, are varied until the calculated pressure curve matches the observed data. The model shows the magnitude of each frictional pressuredrop component at each rate.
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The key requirements for the pre-frac injection test are outlined above. High resolution pressure recording is mandatory. Normal service company resolution of 5 to 10 psi (1 MPa) is unacceptable. The actual rate versus time must also be recorded to compute the average rate, total injected volume, and for any steprate analysis. Start and end of pumping must also be well defined. A minimum shut-in time is required to analyze the injection test. A shut-in time of more than 6 times the injection time gives a G value of >6 which is a minimum to hope to see closure in most reservoirs. Longer shut-ins give better results and are necessary if after-closure analysis is needed. Once closure is confirmed, traditional pressure transient analysis for an impulse test may be used to determine reservoir properties. This is one of the main reasons to specify that the tests be conducted with Newtonian, non-wall building fluids. The reservoir properties in the vicinity of the wellbore are the primary factors that we wish to determine from the pre-frac treatment and not the instantaneous leak-off that we can achieve with various fluid additives. In fact, this is a second issue which will be addressed in the future: If we understand the reservoir then we can evaluate the use of fluid additives to systematically alter the fluid flow characteristics in the reservoir. Nothing precludes the application of this approach other than the time and costs associated with additional injection tests in the reservoir. The first step is to inject a fluid that investigates the basic reservoir characteristics and second is how to modify these properties with fluid additives.