Production of sand from a reservoir is a well known problem in the oil industry. Classically, it is solved by a gravel-pack or frac-and-pack type of completion. This approach, although resolving the sand production problem, has the important drawback of severely reducing the output of the completed well. Recently, two types of completions have been proposed for sanding prone reservoirs: slotted liners for horizontal wellbores and hydraulic fracturing without gravel packing in vertical wellbores. These completions have a less negative impact on reservoir production, but require high quality data acquisition to complete successfully. This paper focuses on how a combination of sonic measurements and stress tests carried out with the micro-hydraulic fracturing technique enabled the design of a successful hydraulic fracturing treatment for sand control offshore California.
Hydraulic fracturing specialists believe that a hydraulic fracture alone can successfully perform sand control if one has proper knowledge of the stresses acting over the zone of interest. The direction of the maximum principal stress is needed to ensure that all perforations are connected to the hydraulic fracture and therefore protected against sand production. Magnitudes of the minimum principal stress are needed for the proper design of the hydraulic fracturing job and, in particular, to ensure proper placement of the proppant which will stop the produced sand from reaching the wellbore.
Such an approach was successfully applied to a sanding- prone turbidite sand-shale reservoir. Critical knowledge of the stresses was acquired by first determining the azimuth of the preferred fracture plane from anisotropy processing of the sonic logs. This direction was validated from local knowledge of the active fault system. It is recognized that stress magnitudes are best measured with the micro-hydraulic fracturing technique. Thus, a special cased hole program was designed to measure the magnitude of the minimum principal stress with a wireline testing tool. Measurements were obtained in the two reservoir sands and three bounding shale layers. These measurements were then used to calibrate a stress log obtained with the processing of a sonic log. This provided a profile of minimum stress magnitudes along the zone of interest.
Analysis of the micro-hydraulic fracturing tests showed very little stress contrast existed between the reservoir rock and the bounding layers. This gave the client an option to design a single hydraulic fracturing treatment for the two layers, more efficient that the two separate treatments initially proposed. The interpretation also confirmed from a hydraulic fracturing standpoint that the azimuth derived from acoustic anisotropy was indeed that of the preferred fracturing plane. This allowed the client to orient the perforations with confidence.
The unique combination of measurements described in this paper enabled the client to design and successfully carry out a hydraulic fracturing treatment for sand control.