Drilling a Downdip Location: Effect on Updip and Downdip Resource Estimates and Commercial Chance

Abstract

A common method for choosing the drilling location for an exploration prospect is to simply drill the well on the crest of the structure. The main driver is to maximize the chance of discovering a hydrocarbon accumulation. However, a common situation for a crestal well is that the discovery of productive reservoir full-to-base with hydrocarbons has only proven a limited productive area. So the discovered resource volumes are too small to justify development and further downdip appraisal drilling is required, adding to exploration costs and delaying possible development. An alternative approach is to choose a well location where a discovery would exceed the minimum commercial field size (MCFS) needed to justify development. In practice, the drilling location is often based on a deterministic resource estimate using the mean value from a probabilistic assessment of resource volumes. A downdip discovery smaller than MCFS or even a dry hole with a thick, porous reservoir might tempt decision makers to sidetrack updip since the full probabilistic distribution of the updip resources had not been considered. This paper demonstrates the advantages of trying to choose a location that considers the full probabilistic resource distributions for the updip and downdip volumes relative to the drilling location and the chance of commercial success in the decision making process. The overlap of the two distributions can be significant and what might seem like a good drilling location based on the deterministic estimate exceeding MCFS actually might leave the decision maker with some regrets at the chosen location. Besides modifying the distribution of discovered resources, the chance of geologic success also changes as the location is moved further downdip. The goals are to maximize the chance of a commercial discovery and to choose a drilling location that if a dry hole occurs there will be no regrets or reason to undertake an expensive updip sidetrack or drill another test updip. To accomplish this, multiple drilling locations need to be evaluated with this process and the results plotted to analyze trends. The results are ready to be inputted into a decision tree for expected value calculations and allow answering management questions such as “If we drill 600 acres downdip of crest, what is the probability we make a commercial discovery and what is probability of leaving commercial resource volumes updip? And how does this compare if we drill only 500 acres downdip?”

AAPG Datapages/Search and Discovery Article #90291 ©2017 AAPG Annual Convention and Exhibition, Houston, Texas, April 2-5, 2017