Abstract

Horizontal wells have been shown to be successful in improving oil recovery for marginal heavy oil reservoirs in Saskatchewan and Alberta. One commonly encountered problem in recovery operations for these unconsolidated reservoirs is the production of sand and fines. The settling and accumulation of the solid particles inside the horizontal wellbore represents a serious problem, with the horizontal well becoming partially plugged being a real possibility. This study investigates this problem, with focus on determining the roles played by different flow parameters on the settling and transport process.

The physical model described in this work examines the transport process mechanistically. Conservation equations for the solid phase (sand particles) and the fluid phase (oil) are formulated, with the interaction between the phases described by empirical correlations. The oil is assumed to be a Newtonian fluid, and the sand particles are assumed to be spherical in shape and uniform in size. The system of equations is solved numerically to determine the distribution of sand particles and oil, and the respective pressure and velocity distributions as a result of the presence of a constriction inside the horizontal well.

According to the simulation results, oil viscosity and particle size play important roles in the transport process, including controlling the gravitational settling tendency of solid particles inside the horizontal wellbore. The results provide insight into the mechanisms involved in the transport process; as such, they provide guidelines for production operations involving horizontal wells in unconsolidated and poorly consolidated reservoirs.

Introduction

One of the most common applications of horizontal well technology in Canada is to recover oil from heavy oil reservoirs in Saskatchewan and Alberta. In heavy oil reservoirs underlain with bottom-water, the use of horizontal wells has been found to improve primary recovery performance prior to water coning - recovering up to 15% of the initial oil in place (in shorter time, also), compared with only 5% for a vertical well. Horizontal wells have also been successfully used for increasing steamflood recovery. However, recovery operations in these heavy oil reservoirs are usually susceptible to sand production due to their unconsolidated nature. The produced sand could give rise to production problems. as the sand fill up the wellbore, prevent the operations of downhole pumps, etc. In the case of horizontal wells, sand production potentially poses an even more serious problem, due to the settlement and accumulation of sand particles at different positions along the well - leading to the reduction of the cross-sectional area of the wellbore open to flow (Figure 1). The ultimate effect of sand deposition could be the partitioning of the horizontal well into several segments, leading to a loss of production and a negation of the principal advantage of horizontal wells (large contact area with the reservoir). The study reported here investigates this problem. Specifically the study examines, through numerical simulation, the transport and distribution of sand particles, pressure, and velocity distributions in a horizontal well. A special feature of the physical model is the presence of a constriction inside the horizontal well.

Solid-liquid flow encompasses many different areas of science and engineering, including the transport of colloids in rain water, sediment transport in river streams, slurry pipeline transportation, drill cuttings removal, transport of proppants in hydraulically fractured wells, etc. The large number of independent variables involved in these transport processes, coupled with the complex interaction between the variables have precluded comprehensive analytical studies of these processes mechanistically. Instead, many experimental studies have been carried out over the years. P. 471^

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