Heave Induced Aquifer Water Level Fluctuations
Authors: Kry P.R. 
Published: March 2010

Abnormal water level fluctuations in Quaternary aquifers have been evident since early 1994 in parts of the Cold Lake Field undergoing Cyclic Steam Stimulation in the Clearwater. In other areas, no responses to steaming operations have been observed. Recent high resolution seismic of the Quaternary has demonstrated a complicated geology, with ice thrusting events leading to isolation of parts of the aquifers into small pockets. Coupling this geological observation with poro-elastic modelling of the effects of Clearwater dilation caused by steaming and subsequent recompaction by production has led to an explanation of the observed water level fluctuations. The mechanism causing water level rises is compression of a pocket of aquifer sand, due to Clearwater dilation as a result of steam injection. For aquifer pockets directly over the steam injection location, aquifer dilation, rather than compression, results from the Clearwater dilation, causing water levels to fall. Aquifer sands that are extensive allow pressure equilibration and no apparent water level changes due to nearby Clearwater dilation. Net injection to the Clearwater appears to be a significant correlative indicator of Clearwater dilation, but sensitivity of the aquifer water level change to net injection decreases with successive steam cycles. First principle numerical simulations coupling fluid flow and formation stress-strain behaviour support the relationships developed.

Introduction

At Cold Lake, Cretaceous age sands of the Clearwater formation located at a depth of 450 m contain bitumen that Imperial Oil Resources has been producing using Cyclic Steam Stimulation(1). These sands are overlain(2) by about 100 m of the Grand Rapids formation, a fluvial interbedded sand-shale sequence, which in turn is capped by the Colorado Shale Group. These marine shales vary in thickness from 150 - 200 m. Quaternary deposits containing fresh water aquifer sands overlie the Colorado Shales. Andriashek et al. have described the general stratigraphic framework for these deposits(3). It is important to know that bitumen recovery operations have not contaminated the aquifers with produced fluids. Therefore, there is an investigation whenever water levels in any aquifer well are observed to behave in an unexpected manner.

Imperial monitors an array of regional groundwater wells. Recent expansion of the operations, into areas closer to some of these wells, has led to unexpected fluid level fluctuations. At the time of occurrence of these fluctuations, extensive inspections of operating CSS wells were undertaken to ensure there were no casing failures, or leaks of produced fluids from wellbores into Quaternary aquifers. Correlation of the water level fluctuations to Clearwater reservoir pressures, as steaming operations moved from one group of wells to another, suggested that highly improbable intermittent sequences of fluid transfers would be required to explain the fluctuations. Any such fluid transfer would have to be via some undetected pathway through 250 - 300 m of overlying sediments. Water chemistry evaluations at an aquifer well near steaming wells confirmed the absence of produced water. An explanation for the anomalous water level fluctuations in terms of stress transfer from the Clearwater to the overlying Quaternary was developed based on concepts from the theory of poroelasticity.