Devising Knowledge Based Decision Tree for Detection of Fracture Corridors From Dynamic Data in a Carbonate Reservoir in Oman
Authors: Ozkaya S. I. , Gordon S. , McFarlane A. , Siyabi S. , Al-Busaidi S. M. , Kramer F. M. , Coskun S. B. , Bolle L. 
Published: December 2007

The subject of this paper is identification and mapping fracture corridors in a carbonate field in Oman using a probabilistic approach. A fracture corridor is a tabular sub-vertical fault-related fracture swarm which intersectes the entire reservoir and extends laterally for several tens or hundreds of meters. The only direct indicator of fracture corridors are borehole image logs. Unlike openhole logs and other borehole measurements, image logs are available only from a limited number of wells. It is necessary to utilize indirect indicators to identify and map fracture corridors, such as lost circulation, step flow profiles, water breakthrough or seismic lineaments. This fits well into a Bayesian scheme to infer the cause from the manifestations. The conditional probability of having a fracture corridor given an indirect indicator such as mud loss is calculated from wells with image logs. Some indirect indicators have no direct link to image logs and a Bayesian inference has to be used to find the conditional probability through an intermediate indicator. For example, the probability of having a fracture corridor given step flow profile is calculated from the conditional probability of having mud losses given a step flow profile, and probability of having a fracture corridor given mud losses. Conditional probability graphs are constructed for continuous variables such as water cut, gross rates and injection rates. Threshold values are defined for these variables such that the probability of having a fracture corridor is more than 0.5 if the indicator is greater than the threshold value.

Good indicators of fracture corridors include mud losses, step flow profiles and water fingering in horizontal wells. High gross rates and water cut are also good indicators but injection rates and sweep in vertical well are not very reliable. The predictive power of indirect indicators improves drastically if two or more indicators favour a fracture corridor. If the wells with fracture corridor indicators are aligned in WNW or NW, the dominant fracture corridor directions in this particular field, the likelihood of having a fracture corridor increases. Final result shows most fracture corridors are located on the southern and northern flanks of the field with a relatively low degree of fracturing in creastal area.

Orientation of fracture corridors from indirect indicators must be estimated from nearby image fracture corridors or seismic faults or from dominant fracture strike in different sectors of the field. The dominant fracture orientation is WNW with one NW fault/ fracture zone on the western flank of this Field. There are also a few NE fracture corridors from short cuts, image logs and well alignment.

An inferred corridor is extended in selected direction (WNW or NW) until it reaches the exclusion zone. Exlusion zones include circle of investigation of well tests with radial flow, horizontal wells with no fracture corridor or a circle around typical matrix producers or injectors. The radius of the circle is determined from the average zone of influence of fracture fairways, which is about 50 m.

The total length of fracture corridors is estimated from image log scan line density. The total number of fracture corridors is estimated from the total length and average length of fracture corridors. The difference between the total number and the number of fracture corridors captured by data analysis is the number of fracture corridors that escaped detection. This number is used to infill stochastic fracture corridors and to generate fracture corridor density and permeability maps.