Dimensions and Degree of Containment of Waterflood-Induced Fractures from Pressure Transient Analysis
- Paul J. van den Hoek (Shell E&P Technology Co.)
- Document ID
- Society of Petroleum Engineers
- SPE Reservoir Evaluation & Engineering
- Publication Date
- October 2005
- Document Type
- Journal Paper
- 377 - 387
- 2005. Society of Petroleum Engineers
- 5.1.2 Faults and Fracture Characterisation, 2.4.3 Sand/Solids Control, 4.1.5 Processing Equipment, 3 Production and Well Operations, 5.4.1 Waterflooding, 3.2.3 Hydraulic Fracturing Design, Implementation and Optimisation, 4.1.2 Separation and Treating, 5.6.9 Production Forecasting, 5.6.4 Drillstem/Well Testing, 5.2 Reservoir Fluid Dynamics, 2.5.1 Fracture design and containment, 5.6.3 Pressure Transient Testing, 6.5.2 Water use, produced water discharge and disposal
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It is well established within the industry that injection of (produced)water almost always takes place under fracturing conditions. Particularly whenlarge volumes of very contaminated water are injected—either for voidagereplacement or disposal—large fractures may be induced over time.
This paper aims to provide a methodology for injection-falloff (IFO) testanalysis of fractured (produced) water-injection wells. Some essential elementsof IFO for fractured water injectors include the closing fracture, (early)transient elliptical reservoir-fluid flow, finite fracture conductivity, andfracture face skin.
An exact semianalytical solution is presented to the fully transientelliptical fluid-flow equation around a closing fracture with finiteconductivity, fracture face skin, and multiple mobility zones in the reservoirsurrounding the fracture. This solution also captures the case that duringclosure, the fracture is generally shrinking from adjacent geological layersunder higher in-situ stress. Based on this solution, type curves of thedimensionless bottomhole pressure as a function of dimensionless time areprovided, covering both the period during fracture closure/shrinkage and theperiod after fracture closure. The shape of these type curves is studied as afunction of the different relevant parameters, in particular the fracturecompliance, the height of in-situ stress contrasts, fracture face skin,fracture closure time, and injection period. It is shown how the fracturelength and height and the degree of fracture containment (in combination withthe heights of the stress contrasts) can be derived from these types of curves.It is also demonstrated that the analyses based on the storage flow and linearformation flow regimes need to be integrated into one analysis method to obtainconsistent results.
Finally, the concepts developed in this paper are applied to a number offield examples, in which the dimensions and degree of containment of theinduced fractures are derived from the analysis of the IFO data.
IFO test analysis offers one of the cheapest ways to determine thedimensions of induced fractures. Unfortunately, hardly any work has beencarried out to date to provide a methodology for interpreting thepressure-transient data of fractured water-injection wells. This contrasts withthe vast amount of work that has been carried out in the area ofpressure-transient analysis for wells with propped fractures. Bothpressure-transient tests during hydraulic fracture stimulation (called"minifrac tests"; see Ref. 1) and pressure-transient tests during productionafter stimulation (i.e., buildup tests; see Refs. 2 through 5) have beenstudied extensively. The theories as developed in Refs. 1 through 5 by now arewell-accepted "textbook" methodologies.
This paper deals with the subject of pressure-falloff analysis on fracturedwater-injection wells. In this area, the situation is entirely different fromthe one above in the sense that until recently, there existed no practicalmethodology dedicated to pressure-falloff analysis on fractured waterinjectors.
The very limited interest in falloff-test analysis on fractured waterinjectors may well be related to the fact that historically, most operatorshave been unaware that their water injectors are fractured. Only in recentyears has this situation started to change. Unfortunately, one of theconsequences of the lack of a dedicated method of analysis is that fallofftests on injectors are generally interpreted in the wrong way, even if onerealizes that they are fractured. Typically, such interpretations lead towellbore-storage coefficients that can be up to orders of magnitude too high,and to fracture lengths based only on analysis of the linear formation flowperiod (see Ref. 10).
The objective of our study is to fill the gap as described above (i.e., toprovide a dedicated interpretation methodology for falloff tests on fracturedwater injectors). In a recent paper, we presented a novel interpretationmethodology for falloff tests on fractured water injectors. This methodology isbased on exact 2D solutions to the problem of pressure falloff around fracturedwater injectors for different boundary conditions.The most important stepforward of Ref. 6 is that it allows the determination of fracture length from aconsistent combined analysis of the storage and linear-to-pseudoradialformation flow periods, and of fracture height from a consistent combinedanalysis of the storage and pseudoradial flow periods. Thus, uncertainties inthe determination of fracture dimensions from falloff-test analysis arereduced.
In the course of analyzing a variety of field cases, we found, based on thesignature of field falloff-test data, that in many cases, the induced fracturesmust have penetrated into adjacent higher-stress zones. Therefore, themethodology as developed in Ref. 6 was extended to cater to this effect, withthe objective being to enable derivation of local in-situ stress contrasts fromfalloff-test interpretation. This extension forms the main subject of thecurrent paper.
The paper is organized as follows. The next section presents thepressure-transient solution for a closing and shrinking water-injectionfracture, including a brief recap of the main concepts presented in Ref. 6. Thethird section presents in some detail the shape of the pressure-transient typecurves for a closing/shrinking fracture as a function of the different relevantparameters, such as the fracture compliance and the height of in-situ stresscontrasts. Subsequently, this method is applied to four field examples.Finally, the last section presents our conclusions.
|File Size||2 MB||Number of Pages||11|
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