Minifrac

Subtopics:

Note:    Minifrac Workflow describes how to use WellTest to analyze data collected from minifrac tests.
Minifrac Observations from Real Data provides insight on what to expect from minifrac tests and highlights the importance of key minifrac test design parameters.

Introduction

Well testing has been used for decades to determine essential formation properties and to assess wellbore conditions. There are many different types of tests that can be used to collect this information depending on when the test is performed, the well location, the well type, and the formation type. For the most part, conventional tests (flow / buildup or injection / falloff) have satisfied the majority of our needs. However, under certain conditions, traditional test methods are not feasible for various reasons. This is especially true for very low permeability formations that require massive stimulation to obtain economic production. For these formations, it is extremely important to establish the formation pressure and permeability prior to the main stimulation. One test that has proved to be convenient for this purpose is commonly referred to as a “Minifrac” test.

A minifrac test is an injection-falloff diagnostic test performed without proppant before a main fracture stimulation treatment. The intent is to break down the formation to create a short fracture during the injection period, and then to observe closure of the fracture system during the ensuing falloff period. Historically, these tests were performed immediately prior to the main fracture treatment to obtain design parameters (i.e., fracture closure pressure, ISIP (Instantaneous Shut-In Pressure) gradient, fluid leakoff coefficient, fluid efficiency, formation permeability, and reservoir pressure). However, since personnel and frac equipment were all waiting on location to perform the main treatment, the falloff period was usually stopped shortly after observing closure, before reliable estimates of formation pressure and permeability could be obtained. Since these two parameters are critical to the fracture design and for production / reservoir engineering, it seemed prudent to extend the falloff period to obtain better estimates, especially since there is little hope of gathering this information after the main stimulation. Many operators have accomplished this by simply scheduling the minifrac test well ahead of the main fracture treatment. However, predicting the falloff time required to obtain meaningful estimates of formation pressure and permeability is difficult, as it depends on having prior knowledge of the permeability, in addition to knowing the geomechanical properties of the formation. In many cases, the progress of a minifrac test can be assessed with pressure data measured at the wellhead, eliminating the need for “guessing” when sufficient data has been obtained.

The created fracture can cut through near-wellbore damage, and provide better communication between the wellbore and true formation, as illustrated below. For this reason, a minifrac test is capable of providing better results than a closed chamber test performed on a formation where fluid inflow is severely restricted by formation damage. The figure below shows an idealized view of an induced fracture.

Typical Pressure Behaviour of Minifrac Tests

The figure below shows a total test overview plot:

The figure below shows typical flow regimes:

Types of Fracture Diagnostic Tests

It is important to acknowledge two operational methods of conducting fracture diagnostic tests.

1. Engineers responsible for designing and completing the main hydraulic fracture treatment prefer to pump the planned fracturing fluid at a high rate / step rate in order to obtain more representative estimates of ISIP, ISIP gradient, net fracture pressure, fluid efficiency and fluid loss coefficients. This information is used to help optimize pad volume, select the best fluid-loss additives for the main treatment, and design the pumping schedule that would create the optimum fracture from a productivity point of view (SPE 140136). Step rate tests can give additional information about matrix leakoff (at low rates) and frac extension (after breakdown at higher rates) (SPE 78173). The higher rates give a bigger minifrac representation and distinct leakoff characteristics. Unfortunately, a bigger minifrac means it will take longer to close. In general, these tests are designed to see closure. In tight / shale formations, such as Eagle Ford and Bakkan, we are not likely to collect enough after-closure data to get good estimates of permeability, or initial pressure based on this type of test.  In unusual circumstances where we can monitor falloff data for a few weeks, we can see reservoir- dominated flow (linear / radial flow) for these tests.

2. Reservoir engineers prefer to inject a less viscous fluid and minimize injected volume in order to obtain enough after-closure data to estimate reservoir permeability and initial pressure. Initial pressure is a key parameter for production data analysis and reserves work for the rest of the life of the well, and it is very difficult to obtain following the main fracture treatment. Leakoff types and coefficients may still be obtained though. However, the relatively small size of the minifrac generated when minimizing injection volume can lead to some uncertainty. For low microdarcy to hundred of nanodarcy permeabilities, we see successful tests performed with rates between 1-2 bbl / min for 5 to 10 minutes, with a total injection volume less than about 25 bbl.

Theoretically, the key parameters can be obtained from either method, but in practice the test objectives must be weighed, and the test designed to meet those objectives. Alternatively, two tests can be performed back-to-back. First, a test can be performed with the injected volume minimized to obtain virgin-rock breakdown pressure, permeability, initial pressure, and initial estimates of leakoff characteristics. Then another test can be performed with the fluid and rate more representative of the main treatment.

Key Results from Minifrac Tests

Following a brief injection period, the wellhead valve is closed, and the pressure falloff is recorded (at the wellhead or downhole) for a few hours to several days, depending on how permeable the formation is. Then analysis of pressure falloff data occurs using specialized techniques to yield the following information:

  • Fracture closure pressure (pc)
  • Instantaneous shut-in pressure (ISIP)
  • ISIP gradient
  • Net Fracture Pressure (Δpnet)
  • Fluid efficiency
  • Formation leakoff characteristics and fluid loss coefficients.
  • Formation permeability (k)
  • Reservoir pressure (pi)

Minifrac Analysis Techniques

There has been much research conducted by numerous experts to establish or advance techniques for analyzing data obtained from minifrac tests. Due to the nature of a minifrac test, the analysis is performed in two parts: Pre-Closure Analysis (PCA) and After-Closure Analysis (ACA). Similar to traditional Pressure Transient Analysis (PTA), specialized time and derivative functions are used to perform PCA and ACA. Variations of these analysis techniques are used in commercial fracture simulation software, which is not convenient for every day use. Recently, the most common minifrac analysis techniques used in the industry today have been implemented in WellTest. When combined with the efficient data management, dynamic wizards, and graphical user interface within WellTest, users now have the ability to easily analyze minifrac test data. An additional benefit gained with WellTest is the ability to advance after-closure analysis beyond diagnostics and straight line analysis to include modeling capability. This provides the advantage of verifying and improving results obtained from diagnostic analyses (similar to the analysis workflow used for many years in traditional PTA).