Injection is a common technique used for several oil filed applications including underground waste disposal, water flooding, Enhanced Oil Recovery (EOR), CO2 sequestration, and others. Depending on the reservoir properties and the injection pressure used, hydraulic fractures can be created. One risk these fractures pose is the possibility that they would breach out of the injection zone through the containment layers. These layers typically overlie the injection zone and may be composed of low-permeability shales, limestones, anhydrites, or others. Loss of containment could result in poor effectiveness in EOR or waterflood applications, or in waste escaping the target disposal formation in the case of waste or CO2 injection. There have been documented cases where the injected fluid has breached all the way back to surface through unconfined vertical fracture growth. The best way to prevent loss-of-containment is through real time monitoring of well performance coupled with well injection tests (such as Pressure Fall-Off and Step-Rate Tests) to determine the reservoir and fracture properties during and after injection.
Knowledge of reservoir properties and fracture dimensions is an essential requirement in the design and operation in fractured injection projects. As these parameters constitute the main input data in the design and monitoring processes a proper methodology must be implemented to evaluate such parameters. Doing so assures that the operation is conducted efficiently and under safe environmental conditions. Well testing and pressure transient analysis are the least expensive methods of estimating the reservoir and fracture parameters. Expert interpretation of this real time and periodic test data can determine if a fracture is being created, its length and height, and if fluid has made contact with faults or natural fractures, and many other properties.
A well test is a flow test that is conducted in order to evaluate formation properties and the existence of a hydraulic fracture. The test is conducted by flowing the well at two or more constant flow rates (usually one of them is zero flow rate). The sand face pressure is recorded with time during the test, and the pressure / time trend can be analyzed to identify different flow regimes and to monitor the formation and fracture properties. Well test can be conducted during the drilling, completion, production, and injection phases. There are several different methods for conducting well tests, depending on the phase of the well’s life.
Production well tests
- Drill Stem Test: A Drill Stem Test is conducted during the well drilling to estimate the formation pressure, permeability, and productive capacity.
- Draw-Down Test: A Draw Down Test is conducted after a well is put on production at constant flow rate after long shut-in. Production pressure can be analyzed to estimate the formation permeability, skin factor, fracture half length, reservoir model, formation extent, and pore volume.
- Pressure Build-Up Test: A Pressure Build-Up Test is conducted by shutting in a production well that produces at constant flow rate. The shut in pressure is recorded and analyzed for formation permeability, skin factor, fracture half length, and reservoir model estimation.
- Well Interference Test: A Well Interference Test is conducted by observing change on pressure of a monitoring well while changing the flow rate on the operating well (producer or injector).
Injection well tests
- Leak-off Test: A Leak-Off Test is conducted during well completion and it is used to determine the pressure at which the rock just below the casing shoe will start to break down.
- Break-down Test: A Break-Down Test is an extended version of the leak-off test. It is used to determine the pressure at which the formation is completely breaks down.
- Injectivity Test: An Injectivity Test is conducted after a well is put on injection at constant flow rate after long shut-in. Injection pressure can be analyzed to estimate the formation permeability, skin factor, fracture half length, reservoir model, formation extent, and pore volume.
- Pressure Fall-off Test: A Pressure Fall-off Test is conducted by shutting in an injector that injects at constant flow rate. The shut in pressure is recorded and analyzed for formation permeability, skin factor, fracture half length, and reservoir model estimation.
- Step Rate Test: A Step Rate Test is conducted by stepping up the injection flow rate and recording the injection pressure for each rate step. Transition from matrix to fracture and fracture propagation pressure can be defined using this test.
Pressure Transient Analysis
The goal of the pressure transient analysis is to determine fracture and formation properties, including: fracture closure pressure (pressure required to open fracture), fracture dimensions, and formation conductivity. These properties can be calculated using injection, production, or shut-in pressure vs time data.
Pressure transient analysis is conducted using a diagnostic plot which graphs the change in pressure (and its first derivative) with time (using a special version of the time variable called superposition time) on a log-log scale. On the diagnostic plot, the plot of pressure derivative becomes flat (constant) when flow into the well is predominantly radial (flowing evenly in all directions in the plane of the perforations). This flow regime corresponds to injection when there is no fracture present. The plot also shows various straight line trends at different characteristic slopes that correspond to other formation flow geometries.
Fracture linear flow (flow from the well into a hydraulic fracture) is seen on the plot when the slope of the pressure derivative is ½ , fracture closure (flow generated as the hydraulic fracture is closed after leaking off all the fluids into the reservoir) can be captured when the slope of the pressure derivative is 3/2, and linear flow (for example, flow from the well in one direction into a fracture connected to the well_) is observed when the slope of the pressure derivative is ½.
There are many additional flow regimes which can be read by an expert or with specialized software for diagnosing well flow patterns. Moreover, when pressure transient analysis is conducted at regular intervals, say, quarterly or yearly, trends and changes in the flow regimes can be captured. This type of regular monitoring is a critical enabler of safe, sustained, long term injection.
Fig. 1: Sample for Log-log diagnostic plot for data (after Mohamed et al. 2011)
Article written by HEI contributor Omar Abou-Sayed.
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