Horizontal Drilling In the Keg River Carbonate Reefs
Authors: Chernyk J.F. 
Published: March 2010

Abstract
This paper describes techniques and drilling practices optimized through drilling six medium radius horizontal wells, in carbonate reefs, under vertical miscible flood in the Rainbow Lake area of N.W. Alberta, Canada.
Horizontal drilling technology was utilized to alleviate gas/solvent coning through reduced drawdown. This will accelerate and increase oil recovery beyond that achievable with vertical producers.
Drilling techniques were optimized by a continuous process of technique evaluation and improvement. Several monitoring methods were developed, permitting real time evaluation of drill-string drag, which proved to be effective in monitoring hole cleaning and the onset of wellbore solids coating. Well design, reef penetration procedures, bottom-hole assembly choice, drilling fluids and coring techniques have been optimized over the six well program, such that each successive well met its technical and economic objectives.
Average production of the horizontal wells was 2.8 times that of vertical wells with lower drawdowns. Through the optimization process, well costs were reduced from 25 times that of a vertical well to 1.7 times.

Introduction
The Rainbow Basin is located in a remote area of northwestern Alberta, Canada at 58 °N latitude and 114 ° W longitude, with an areal extent or approximately 30 square kilometres (Figure 1). The surface topography is muskeg and stunted coniferous forest. Temperature varies from -50 ° C in winter to +30 ° C in summer months. Oil field activity is seasonal due to restricted access, hence most drilling activity commences upon freeze-up in October and continues until spring thaw in early April. Because of the random nature of pinnacle reef structures, concentrated development efforts further complicate access with busy roads and pipeline networks. Husky Oil operates 250 of the 700 wells drilled in the Rainbow Basin. The field was discovered in 1965, with secondary recovery techniques starting in the early 1970's. The primary oil objective is the Devonian Keg River pinnacle reef structures. Husky Oil operates seven vertical miscible floods in the Rainbow Lake area. As these miscible floods reach maturity, gas/solvent coning in the vertical producers has become a major problem.
FIGURE I: Rainbow basin - major Keg River pools. (Available in full paper)
The objectives of horizontal well technology in Rainbow are to alleviate gas and solvent coning by reducing drawdown and to increase the recoverable oil by producing a higher percentage of the oil sandwiched between the gas and water zones (Reference 1, Figure 2).

Geological and Reservoir Characteristics
Since all pools have similar characteristic, the "A" Pool structure will be discussed for the purposes of this paper. The "A" Pool structure is a large pinnacle reef with an area extent of 467 hectares, extending approximately 2,200 m in diameter at the original oil/water contact, 750 m in diameter at the reef top and 220 m thick. The reef consists of an inner reef core and reef fore-slope comprised of 100% dolomite. The inner reef core has vugular and inter-crystalline porosity with high permeability of 12% and 100 md respectively. Lost circulation problems are common when drilling into this section of the reef, due to large vugs and possible fractures.