Time-dependent Effect of Drilling Fluids On Fernie Shales
Authors: Wan R.G. , Achari G. , Schacter R. , Joshi R.C. , McLellan P.J. 
Published: March 2010

Abstract
This paper investigates the time-dependent effect of drilling fluids on the shear strength properties of Fernie Formation shales. Direct shear tests were conducted on samples that were split along their bedding planes and soaked in four different drilling fluids systems for varying periods of time: oil based mud, gel chem, TAME and PHPA. Distilled water was used as a base line fluid in this study. The shear tests were conducted at a normal stress range of 0 – 1 MPa. For each fluid, five samples were tested, four of which had their natural rough surface, whereas, the surface of one was artificially smoothened.
The test results show that both TAME and PHPA fluids increase the angle of shearing resistance of the smoothened shale samples indicating that these may be absorbed into the shales, at least at the surface. The distilled water, oil based and gel chem muds have no effect on the angle of shearing resistance of smooth shale samples, whereas, the angle of shearing resistance of the rough samples decreased. The angle of shearing resistance of the rough samples in the TAME fluid showed both increases and decreases, whereas rough samples in the PHPA fluid had a general increase in the angle of shearing resistance with period of soaking.

Introduction
Shales from the Cretaceous and Jurassic periods are found at depths of thousands of metres in the Rocky Mountain Foothills region of British Columbia and Alberta, Canada. In undisturbed conditions, these shales exist in equilibrium under high in situ stresses. However, when drilled for oil and gas, these shales tend to become unstable in boreholes(1,2,3). This loss in stability is due to the mechanical stresses induced while drilling as well as the physico-chemical effects of the drilling fluids. The relative contribution of each of the two effects in causing instability is not well understood in some brittle shales like the Fernie Formation. Mechanical stresses are caused by the excavation of the wellbore, whereas some physico-chemical effects are the result of capillary action due to absorption of the fluid and interaction between the constituents of the rock and that of the fluid(4).
To our knowledge, the physico-chemical effects of the various drilling fluids on Fernie Formation shales have not been studied in the laboratory. Hence, a study was initiated to determine the shear strength parameters of Fernie shales along pre-failed bedding planes when exposed to different drilling fluids for varying periods of time. The time-dependent effect of the fluid on the shear strength properties of the shales was examined by conducting direct shear tests on samples soaked in the fluid for periods of one, seven and 28 days.
There are several different parameters that affect the shear resistance of natural rock discontinuities. The surface roughness and interlocking of protrusions (asperities) are considered major obstacles in objectively determining rock discontinuity shear strength. The problems are compounded by the apparent effects of fluid induced lubrication including softening of asperities, fluid composition, rock mineral content, base frictional properties and normal stress levels.