Production Enhancement Strategies for Strong Bottom Water Drive Reservoirs
Authors: Ehlig-Economides C.A. , Chan K.S. , Spath J.B. 
Published: December 2007

Severe water coning occurs when producing oil and gas from a strong bottom water drive reservoir. This paper compares and evaluates several new drilling and well completion concepts designed to optimize production and delay water coning.

For basic types of vertical well completion (partial, total, reverse and dual penetration) strategies are first examined using reservoir data from a field being developed. Although high water rate production cannot be avoided in all cases, oil rate and cumulative recovery can be significantly improved using total and dual penetration methods. Gel treatment to build a coning barrier was found to be ineffective and uneconomical.

Under the same reservoir conditions, alternative options such as fracturing, horizontal well and short radius laterals were also evaluated. Multiple laterals that can be implemented with current drilling technology offer a promising strategy for delaying water cresting and enhancing production.

Introduction

For understanding well production performance in a reservoir under strong bottom water drive, nothing is more revealing than observing the near-well fluid flow geometry and pressure distribution under different well drilling and completion schemes.

Some of the fluid flow constraints and restrictions are inherited. Some are artificially created by the well drilling and completion scheme. This entails selection of well location, well spacing, type and extent of completion and perforation. These inherited and imposed constraints effect the pressure drawdown distribution near the well. In turn, a near-well fluid flow geometry develops progressively.

For vertical wells open to flow only in a portion of the oil zone and for horizontal wells drilled above the oil-water contact, water from the underlying aquifer moves upward to a characteristic shape. For these strategies, the key to successful reservoir exploitation is to maximize the volume swept by the rising water by manipulating the flow geometry.

By virtue of the shape of the swept volume under the rising water, breakthrough at the perforation interval is called coning in vertical wells and cresting in horizontal wells. This adverse water entry at the wellbore impedes oil and gas production. It bypasses the hydrocarbon reserve, modifies the oil and gas contact with the wellbore, and increases the wellbore hydrostatic pressure which reduces the bottom-hole flowing pressure and hence, the drawdown pressure. In addition it can generate formation damage such as wettability change, water block (high water saturation), emulsion and scale deposition.

The maximum water-free oil production rate is called the critical rate. A great number of authors have conducted experimental, analytical and numerical studies on coning behavior in vertical wells. Based on empirical or analytical equations, these studies neglected some key factors such as varying bottom water influx rate and pressure, and viscous/gravity and capillary pressure effects, and they predict generally low and uneconomical critical coning rates and critical drawdown pressure. The controlling parameters are either geometrical, namely the effective wellbore and drainage radii, the location and length of perforation interval, and the distance of deepest perforation to water/oil contact (WOC) or physical, namely the hydrocarbon/water density contrast and the ratio of horizontal to vertical permeability.

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