Fracture Closure Stress Treatment in Low Fracture Pressure Reservoir
Authors: Lai Yau Kuan , Woodward Bruce 
Published: March 2015

This paper describes a Fracture Closure Stress (FCS) treatment to increase the fracture pressure of a significantly depleted sandstone reservoir from 7.3 ppg to 10.5 ppg in a cased-hole application. This treatment was executed on a gas well open to a highly depleted zone of 2.1 ppg and a higher pressured zone of 7 ppg. Previous attempts to fill the well with kill fluid had been unsuccessful due to high seawater loss rates. Efforts to reduce losses by pumping lost circulation material had also been unsuccessful. A FCS treatment was designed to initiate a fracture in the low pressure interval and isolate the tip of the fracture with pill material while allowing the fracture width to grow to increase FCS as needed to control the well. A high-solids content, high-fluid loss pill was displaced with seawater to replicate a hesitation squeeze until the well was able to stand full for a conventional hesitation squeeze.

The information in this paper may assist in the planning and execution of loss treatments in other workover applications where the fracture pressure is low and multiple reservoirs of differing pressures are open in a single wellbore.

FCS was successfully increased to 10.5 ppg. This successful treatment allowed the well to be filled with kill fluid for all zones open in the well with adequate margin to prevent fracturing to allow rig operations. This paper describes the treatment, execution, challenges, and potential applications of this technology.

This paper may assist in controlling wells where the fracture pressure in a zone is low, with other intervals of differing pressure open to the well at the same time. This can be used to successfully prepare such wells for rig operations. It validates the application of this open-hole drilling technology in cased-hole applications.

Introduction

Loss returns in a wellbore can be differentiated between four main mechanisms; matrix losses, fracture propagation losses, conductive fracture losses, and vugular losses. The mechanism of interest for this subject is fracture propagation losses, where losses in a well occur at high rates when pressure in any part of the wellbore exceeds the fracture pressure of the formation. The Fracture Closure Stress (FCS) concept describes the mechanism by which a fracture in a wellbore initiates and propagates. The FCS concept describes that a wellbore has stress holding the wellbore closed. The stress around the wellbore comes mainly from the lateral translation of force from the overburden stress and also from the pore pressure of the formation.1 Whenever the stress holding the wellbore closed is exceeded by the pressure inside the wellbore, fracture initiates and propagates. The effect of rock tensile strength on fracture pressure cannot be accurately depended on, and is likely to have little effect in most cases.1,2,3 With this, the wellbore can be thought of as a reliable pressure relieve valve, which opens up whenever stress (or pressure) inside the wellbore exceeds the stress holding the wellbore closed (the fracture pressure).1 The stress holding a wellbore closed can change over time. At a certain time reference, the overburden at a depth is supported by both the formation and the fluid within the formation (pore pressure). As fluid in the formation is produced and pressure depletes, more of the overburden is supported by the formation versus the fluid in the formation. While fluid translates pressure equally in all direction, solids do not. The amount of stress translated in the perpendicular direction of a solid object depends on the Poisson's ratio of the rock.4 When a reservoir depletes, and more of the overburden is supported by the formation and less is supported by the fluid in the formation, less of the overburden stress is translated to the lateral plane, decreasing the stress holding the rock closed. This describes the reduction of fracture pressure as reservoir pressure depletes.3,4