Abstract

Reservoir wettability characterization is often important for planning and predicting depletion strategies, including primary and enhanced recovery schemes. This paper compares wettability evaluations of water-advancing contact angle (WACA) using the recently developed "Dual-Drop- Duo-Crystal (DDDC)" technique(l) with the traditional "modified sessile drop" procedure. The DDDC technique was found to be a more reliable method for the Elk Point crude oil/brine/rock systems. The DDDC experimental procedure is less time consuming because this technique achieves rapid adsorption equilibrium between two crude oil drops and two crystal surfaces. Moreover, if is reproducible as it is designed to overcome the difficulties commonly associated with the traditional contact angle methods.

On the basis of the DDDC contact angle experiments, conducted in 1993, it was concluded that both the Lindbergh and Frog Lake Cummings heavy oil reservoirs of the Elk Point field exhibit intermediate wettability near the native resen10ir conditions (characteristic of primary depletion). The water-advancing contact angles for the crude-oil//brine/quartz systems (near the reservoir conditions of 4400 kPa and 28 °C) were determined to be 92 °and 90 ° for Lindbergh and Frog Lake, respectively. These angles were reproducible within 1 °. This reproducibility is the result of meticulous application of water-advancing contact angle definition by means of a procedure that recognizes the effects of adhesion and buoyancy and allows shifting of the lower crystal so that the brine advances over an area that was previously exposed to crude oil. In addition, careful monitoring of three-phase (oil/water/solid) contact line (TPCL) movements ensures conformance to the definition of water-advancing contact angle. Contact angles at elevated temperatures were also measured using the DDDC technique. The results showed that the Frog Lake system shifts to oil-wet behaviour at a temperature of 97 °C or higher.

Introduction

Reservoir wettability characterization is important to nderstand fluid flow, recovery mechanisms and production behaviour of petroleum reservoirs containing either light or heavy oil. A knowledge of reservoir wettability is also required to design drilling and completion programs to avoid formation damage as well as under-balanced drilling, particularly for horizontal wells. Reservoir wettability plays an important role in such tertiary recovery processes as gas injection (miscible/immiscible) and chemical flooding as well as thermal EOR processes for heavy oils.

The primary purpose of this paper is to analyze, evaluate and compare experimental procedures as well as contact angle measurement results of the modified sessile drop technique with those of the "Dual-Drop-Dual-Crystal" (DDDC) technique(l) based on the tests conducted on the Lindbergh Cummings heavy oil crude-oil brine/rock system. The DDDC technique was recently developed by the Petroleum Recovery Institute (PRI) to improve contact angle wettability characterization. Results from the DDDC technique for the Frog Lake heavy oil system at native reservoir conditions as well as at elevated temperatures will be discussed. First, a brief overview on reservoir wettability as currently encompassed by reservoir engineering practice is provided to give a perspective on the importance of wettability characterization (for laboratory and field practices), significance of water-advancing contact angle (WACA) definition and the status of wettability knowledge for heavy oil reservoirs.

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