Optimization of Horizontal Well Placement
Authors: Wagenhofer T. , Hatzignatiou D.G. 
Published: December 2007

In this study the coning time of water and gas as well as the optimization of a horizontal well placement with respect to the gas-oil and water-oil interfaces are examined. A three-dimensional, three-phase, black-oil commercial simulator is used to simulate the simultaneous movement of the gas-oil and water-oil contacts as a function of time towards a horizontal well located in the oil zone. A parameter sensitivity analysis is conducted to evaluate the influence of various factors and reservoir rock and fluid properties such as fluid viscosity, fluid density, reservoir relative permeability curves, formation anisotropy, horizontal well offset from the water-oil contact, etc. on the breakthrough time and the optimum horizontal well location. Correlations for predicting the time at which gas and water cone simultaneously into a horizontal well and the optimum location of the well with respect to water-oil and gas-oil contacts are developed.

Introduction

Gas and/or water coning encountered in many oil wells is a serious problem which results in lower oil production rates, lower oil recovery, increased lifting cost and ineffective solution-gas drive mechanisms. The main forces acting on the reservoir fluids besides capillary forces are gravity and viscous forces. Gravity forces are due to the density differences of reservoir fluids such as water-oil and gas-oil and viscous forces are due to fluids production and wellbore drawdown pressure. The pressure gradient forces, which depend on the oil production rate, tend to lift the water- and/or gas- oil interface in a direction perpendicular to the formation bedding plane, whereas the gravity forces tend to separate the oil, gas and water phases. When the viscous forces overcome the gravity forces the gas-oil and water-oil interfaces start to move toward the production well until they reach the producing well. This phenomenon is referred to as coning or cresting in vertical or horizontal wells, respectively, and the time at which the unwanted fluid (water or gas) reaches the producing well is termed breakthrough time.

Traditionally, due to technological restrictions and economic considerations, wells were drilled mainly vertical to the oil bearing formation. However, due to recent advances in oil-well drilling technology and a decrease in drilling cost, horizontal wells are now being drilled routinely to drain oil reservoirs. One of the main advantages of the horizontal wells compared to vertical wells is that it reduces pressure depression in the well's neighborhood due to the smaller flow rate per unit length of the well for a given production rate. Therefore, horizontal wells tend to attract the fluid contacts more uniformly over a larger area than vertical wells and, therefore, decrease the tendency of the water and gas to "cone" into a producing well. Especially in thin oil zones, located between the bottom water and gas cap, horizontal wells have substantially reduced the coning effects.

The intensity of the coning phenomenon generally depends on the pressure drawdown in the neighborhood of the producing well, the viscosity and density of the water and gas compared to the oil phase, and the vertical and horizontal formation permeability. The time at which water or gas reaches the producing well can be delayed by increasing the offset of the well with respect to the WOC or GOC, respectively. In this case, however, because the oil zone exists in between a water and a gas zone, the water and gas coning times generally will be different if the horizontal well is designed so that the offset of both WOC and GOC is maximized, i.e., the well is placed at the center of the oil zone. This is because the mobilities and densities of the water and gas phases are significant different. It is intuitively apparent that there would be a location within the oil zone at which both water and gas fluids cone simultaneously into the horizontal well; at this location the pre-breakthrough time, cumulative oil production is maximized.

Knowledge of the breakthrough time is very significant for effective oil well management and for extending the oil production time of the well without the presence of water or free gas. The importance lies in the fact that once water or gas has broken through, the fluid distribution and the fluid relative permeabilities in the system will change. P. 629