Analysis of Swelling and Softening of Shale (Colorado Group) Near a Drilled Hole Subjected to Water Invasion
Authors: Wong R.C.K. 
Published: March 2010

This paper presents a mathematical model for the analysis of a time-dependent swelling-softening process in clay shale around a drilled hole subjected to fresh water invasion. The model is derived from a set of governing equations involving equilibrium and mass balances for fluid and ions components. Flow laws, according to hydraulic, repulsive, and osmotic pressure heads, are required. The effect of modulus reduction due to swelling must be also taken into consideration. The proposed model was used to simulate the experimental results observed in hollow cylinder tests on a Colorado Group shale. A computer tomography scanning method was used to examine the deformation of the hole for a period of 72 days. In addition, the amount of sodium chlorine ions released from the shale specimen into the water was monitored continually. Analysis of the test results show that the swelling and softening mechanism is a coupled mechanical-hydraulic-chemical process.

Introduction

Shale (mudstone), the most common of all rocks, has long been known for its tendency to swell upon adsorption of water and lose its structural integrity due to swelling. Water-sensitive shale formations cause costly and time consuming problems in the drilling and completion of wells in the petroleum industry(1). In the past, extensive efforts to characterize the engineering properties of swelling shales have been conducted by many researchers(2–6). Most of the previous research on shale was performed based on element tests in which the stress-strain relationship, or swelling properties, were determined. Few attempts seem to have been made to conduct special tests simulating the stress paths encountered in drilling. This paper presents test results of swelling and softening in Colorado shale around a drilled hole subjected to fresh water invasion. In addition, a mathematical framework has been developed for analysis of the complex boundary-valued problem, i.e., the time-dependent swelling and softening mechanism around the drilled hole. The parameters defined in this framework must represent the swelling physics within the shalewater system, and these parameters are measurable in the laboratory. Interpretation of the testing results provides critical insight into the response of shale subjected to environmental changes.

Testing Procedures and Results

The shale cores for this experimental study were recovered Cold Lake, Alberta. Core sampling was carried out using a conventional rotary core barrel of 89 mm inside diameter. The shale cores were from a drilled section (24A) of a depth of 210 m from the surface, and 40 m into the Colorado group. This group of formations has widespread exposure along the eastern base of the Front Range of the Rocky Mountains. Further east, the Alberta Shale of central and southern Alberta includes the Colorado group and the overlying Lea Park Shale. In northern Alberta the Colorado Formation is represented by all but the uppermost beds of the Smokey Group, as well as the Dunvegan, Shaftesbury, and Paddy Formations, and essentially all of the Labiche Formation(7). The Colorado shales at Cold Lake have a fairly consistent mineralogical compositions. The sieve analysis data are: 1 – 5% of sand, 42 – 48% of silt, and 47 – 57% of clay.