Use of Uncertainty Analysis in Evaluating Hydrocarbon Pore Volume in the Rainbow-Zama Area
Authors: Pritchard K.C.G. 
Published: December 2007

A technique, including Monte Carlo simulation and history matching, for evaluating hydrocarbon pore volume in an environment of uncertainty is demonstrated through a case history study of a specific Rainbow-Zama pool in Alberta.

Introduction

Oil was discovered in the Rainbow Lake area of northern Alberta in 1965 and the play extended north to the Zama Lake area in 1966. By the end of 1968 some 150 separate pools had been discovered, with total oil in place approximating 2.1 billion bbl, of which 800 million bbl is estimated as recoverable under the producing mechanisms now in effect. The play is by no means exhausted and could extend both to the northwest and to the southeast. It is conceivable that the Rainbow-Zama area of northern Alberta will ultimately be found to hold 5 billion bbl of oil, 3 billion of which will be recovered through various recovery schemes.

The features exist as pinnacle reefs that grew on a common platform.5 There is an extreme range in the size of the reefs: the hydrocarbon pore volume varies from 500,000 to 500,000,000 STB. In general, the larger features have been found in the Rainbow area.

The seismograph has proven to be particularly successful in the Rainbow-Zama area. Modem techniques, including common-depth-point stacking, have located the crestal portion of the pinnacles with almost 100 percent success. The areal extent of the reefs and the size-shape relationship are not so easily defined by seismic methods. A few wells, drilled on development locations in the flank positions, either did not encounter the reef or encountered it below the oil-water contact. This presents a paradoxical situation in which the risk associated with an exploratory well is essentially nil and the risk associated with a development well is high. As a result, development wells have been drilled only where the seismic data show the feature to be large. There are few off-reef wells, and many of the features are one-well pools, penetrated only by a crestal well.

In Alberta, producing capability exceeds market demand. For this reason, a production allowable is established for each well in the Province by the Alberta Oil and Gas Conservation Board. This allowable is prorated on the basis of the recoverable reserves. Initially, the Conservation Board assigned Rainbow-Zama allowables by assuming that the wellbore parameters were representative of the 160-acre drilling spacing unit. Seismic data indicated the features to be essentially circular, although seismic estimates of the exact reef flank position and side-slope angles proved to be unreliable. Fig. 1 is a schematic of the four gross models that could be deduced from the seismic data and initial wellbore evidence. The range of uncertainty is extreme. For example, the feature could conceivably vary between a cone and a cylinder, with the conical volume being one-third the cylindrical volume. The most likely geometric configuration is that of the frustum of a cone with side slopes of 30° to 45°.

Most of the available acreage in the Rainbow-Zama area was competed for under bonus bid strategy. Bids were generated on the postulated reserves of an individual feature, along with the best estimate of the allowables to result from such reserves, overlaid on a most reasonable decline in production capability.