An Unexpected Benefit of Horizontal Wells On Offset Vertical Well Productivity In Vertical Miscible Floods

Abstract

Since 1989, Husky Oil has drilled 12 horizontal wells in six mature hydrocarbon vertical miscible flood pools. To date, these horizontal wells have been very successful in reducing gas coning and improving oil productivity(1) as originally intended. Furthermore, actual field performance indicates that, in some cases, horizontal wells have considerably improved productivities of adjacent vertical wells. The drilling of a horizontal well in the Rainbow Keg River E Pool resulted in a 10 –15% increase in oil production and a cessation in the rising gas-oil ratio in the offset vertical wells. Such unexpected benefits are attributed to improved drainage to the vertical wells induced by the drilling of the horizontal well, along with the healing and/or reduction of the gas cone at the vertical wellbores. These hypothesized causes for the unexpected benefits were qualitatively confirmed with a 3-D numerical simulation model. It is concluded that drilling of horizontal wells can improve the productivities of offset vertical wells. The fear of drawing gas over to a horizontal well that is drilled close to gas coning vertical well(s) may be unfounded under most situations. After drilling a horizontal well into a target area, the lateral section can be extended to cover areas near offset vertical well(s) to improve drainage and reduce some of the gas coning tendencies around the offset vertical wellbore(s). Introduction Since 1989, Husky Oil has drilled 12 horizontal wells in six mature hydrocarbon vertical miscible flood pools (Figure 1). The original intent of the horizontal drilling technology in these reservoirs is two-fold:to minimize coning and channeling of gas and water,to attain larger drainage area, resulting in incremental oil sandwich recovery and improved oil productivity(1–3). To date, all of these horizontal wells have been extremely successful in reducing gas coning and in improving oil productivity(1). On average, initial capital investment is paid out within one year. The length of the lateral sections has increased from 178 m in the first well to as long as 720 m with each subsequent well drilled into a progressively riskier target(1). More importantly, actual field performance now indicates that, in some cases, horizontal wells can also considerably improve the productivities of adjacent vertical wells. This paper presents one case history (Rainbow Keg River E Pool) showing the additional benefits of horizontal wells, the results of a conceptual numerical simulation model that was used to rationalize and substantiate the reasons for such unexpected benefits, and finally a discussion on a new application of horizontal well technology in further enhancing oil recovery in mature, vertical miscible floods. Field Performance Observations Rainbow Keg River E Pool Pool Description The Rainbow Keg River E Pool (Figure 1) is one of the carbonate reefs of Middle Devonian age discovered in the mid-sixties in northwestern Alberta. Using both material balance and volumetric calculations, the original oil-in-place (OOIP) was determined to be 4.2 × 106 m3.