Productivity Enhancing Wettability Control Technology For Cyclic Steam Process In the Elk Point Cummings Formation
Authors: Rao D.N. , Karyampudi R.S. 
Published: March 2010

The phenomenon of wettability shift to an oil-wet behaviour and reduced relative permeability to oil at high temperatures due to steam injection in the cyclic steam (CS) process was identified as one of the factors responsible for the less than expected performance at the Elk Point Cummings formation. This paper reports on the results of a laboratory investigation and on the discovery of a method as well as its field implementation to prevent wettability shifts at elevated temperatures. The contact angle experiments revealed that a wettability transition to an oilwet state occurred at elevated temperatures in the rock-fluids system consisting of a quartz crystal surface, Lindbergh crude oil and synthetic brine that represented a mixture of steam and formation water. A mechanism is proposed to explain temperature dependence of reservoir wettability by applying a Zismantype correlation and thin wetting film stability considerations.

It was discovered during this experimental investigation that the wettability reversal at elevated temperatures could be prevented. As the temperature in the contact angle cell was increased, calcium carbonate (CaCO3), due to its decreasing solubility at higher temperatures, precipitated onto the quartz crystal surface. This deposition of calcite particles on the crystal surface imparted an oil resisting nature and a strongly water-wet behaviour to the surface even at elevated temperatures. The effectiveness of this discovery on improved oil recovery was further confirmed through core floods with and without CaCO3 precipitation. The core floods also enabled the determination of a suitable technique for depositing CaCO3 by injecting separate slugs of sodium bicarbonate (NaHCO3) and calcium chloride (CaCl2). The discovery was then field tested in an Elk Point cyclic steam pilot well. This paper reports, for the first time, both the laboratory and field results of the wettability control technology for productivity enhancement in thermal enhanced oil recovery.

Introduction

Considering the enormous resource base of 425 ??109 m3 of heavy oil and bitumen in western Canada, Miller (1994) concludes that the oil production business will eventually be working mainly with heavy oils and bitumens. The Lloydminster heavy oil belt alone is estimated to contain 3.1 ??109 m3 of oil-in-place with API gravity ranging from 16 to 24 ° and viscosity in the range of 100 to 400 mPa.s at reservoir conditions (Polikar et al., 1993). Cummings is one of the extensive formations in the Lloydminster heavy oil belt and contains a 13 ° API heavy oil with a stock-tank oil viscosity of about 10,000 mPa.s at reservoir temperature of 24 ° C. The immense magnitude of the original oil-in-place makes the Cummings reservoirs prime candidates for improved oil recovery processes with potential for significant economic benefit through incremental oil recovery over primary production.

PanCanadian's experience with good primary production as well as the reservoir response to cyclic steam (CS) pilot tests at the Elk Point Cummings formation have been recently presented (Karyampudi, 1995).