Applications of Wireline Stress Measurements

Summary

This paper describes applications of wireline stress measurements based on the microhydraulic fracturing and packer fracturing techniques.

New testing and interpretation procedures have been developed. These procedures make it possible to obtain reliable measurements of the minimum horizontal stress and also the maximum horizontal stress in a near-vertical openhole environment, or to conduct tests in a cased-hole environment.

Examples of stress tests carried out in a wide range of formations are presented, with applications to the design of hydraulic stimulations, the stability of deviated wells, screenless completions for sand control, and enhanced recovery programs.

Introduction

The need for more detailed stress measurements has increased over the past decade. Knowledge of the value of the minimum principal stress and possibly its orientation used to be the only input the oil and gas industry required in term of stresses. Today, more applications benefit from information on the complete stress tensor (i.e., the value of the three principal stresses and their orientation).

The goal of stress measurements is to measure the value of the minimum principal stress ?₃ and possibly its orientation. The measurement should also yield values of the "far-field" stresses (i.e., undisturbed by the wellbore). More detailed measurements should yield the value of the intermediate principal stress ?_int and possibly that of the maximum principal stress ?_max together with their orientation. Various techniques have been proposed to measure the in-situ stress (for a review, see Ref. 1). These techniques include overcoring, analysis of focal mechanisms of induced seismicity, size of breakouts, and core relaxation (differential strain curve analysis and anelastic strain recovery). The stress measurements presented in this paper are based on the microhydraulic fracturing technique. It is the best-known technique to measure stresses at great depth,² although it can be used in conjunction with other techniques for added completeness.³ This technique uses the pressure response obtained during the initiation, the propagation, and the closure of a hydraulic fracture (Fig. 1).

This technique has been most successful in measuring the value of the far-field minimum stress in low-permeability rocks. In this paper, we report solutions to the challenges posed by extending stress measurements based on the microhydraulic fracturing technique to test both impermeable and very permeable formations and to access more information about the stress tensor than just the value of the minimum stress.

After some theoretical considerations, we describe the hardware and report the developments in testing procedure and test interpretation. Then, for each new application, we list the corresponding challenges and the proposed solutions, followed by an example.

Note that the analysis presented here is based on the framework of linear elasticity. Recent work^4,5 has shown that nonelastic behavior of the formation may yield to estimates of the stresses that are too low.

Testing a Fracture to Measure Stresses

Let us first mention the relation between a fracture and a wellbore placed in a medium subjected to a triaxial stress field.

Closure Stress and Minimum Principal Stress.

Let us consider a fracture filled by fluid at a constant pressure and loaded by a constant stress. The width of the fracture is zero if the pressure of the fluid in the fracture is equal to that stress. If the stress acting on the fracture surface is not constant, the constant fluid pressure at which the fracture opens or closes at the wellbore is a weighted average of that stress. The value of this pressure is called the closure stress. If most of the fracture surface is loaded by a constant stress, the closure stress is a good estimate of that constant stress. The goal of stress testing with the microhydraulic fracturing technique is to develop a fracture with a closure stress as close as possible to the far-field principal stress ?₃ for which we seek to determine the value.

Closure stress cannot, however, be measured because constant fluid pressure in the fracture cannot be achieved in practice. What is measured is the pressure at which the fracture opens or closes at the wellbore during hydraulic tests. It is detected by looking at abrupt changes in the flow regime associated with the abrupt changes in fracture conductivity when the fracture opens or closes. For this pressure to be as close as possible to an estimate of the closure stress, it is desirable to measure fracture closure or reopening when the fluid in the wellbore is as close as possible to the pressure in the bulk of the fracture. This is best achieved when the fluid injection (or withdrawal) rate is zero.

These considerations led to development of the procedure described in more detail in this paper. A hydraulic fracture is repeatedly tested and extended. At each stage, various estimates of the closure pressure are determined. Reconciling all estimates for all stages leads to estimation of the closure stress. Finally, other information such as the density of the sediments or wellbore images can be used to assign the estimated closure stress as an upper bound, lower bound, or fair estimate of a far-field principal stress.

Equipment

The tool used to perform the stress tests reported in this paper is the wireline-conveyed MDT* modular formation dynamics tester. This tool, which uses motorized valves, is of modular design and entirely software controlled. The configuration used for stress testing includes the packer module, pumpout module, and flow control module⁶ (Fig. 2).

The packer module isolates a 3 ft section of the wellbore for testing. Pressure in the packers and in the test interval is recorded simultaneously and can be displayed for analysis in real time.

The pumpout module can be used to pump fluid from the mud column either to the packers or to the interval. A new pump was developed specifically for stress testing. This pump delivers a maximum flow rate (about 3 L/min) if the pressure difference between the inlet (mud column) and the outlet of the pump (packer arrangement or interval) is less than 150 psi for quick packer inflation. It then delivers a smaller flow rate independent of the differential pressure for better real-time control of the tests. That flow rate can be controlled by changing the speed at which the pump operates to a maximum of 1.1 L/min.

The flow control module is used to withdraw fluid from the interval at a constant flow rate.