Is Water Cut Influenced by Natural Fractures in South of Sultanate of Oman Porous Clastic Reservoirs? An Answer Based on Integrated Observations
Authors: Al-Sulaimi Ghadna , Bazalgette Loic , Al-Shuaili Salim , Al-Harthi Mohammed , Al-Shuaibi Fakhriya , Large Samantha 
Published: October 2022

The field location is in the South of the Sultanate of Oman. It produces from two clastic reservoirs which are mainly developed by closely spaced horizontal wells. One of the key challenges is the quick development of a high water cut. During the recent past years, this behavior was attributed to water shortcutting through a supposed network of natural open fractures in the reservoir. For this reason, the usual mitigation consisted in acquiring borehole images (BHI) to detect supposedly natural open fractures. The corresponding well sections were then isolated using swellable packer (EZIP) technology to delay water cut development. Questioning the presence of an abundant network of open natural fractures in the reservoir and their potential role in high water cut, an integrated technical study was performed to evaluate the adequacy of data acquisition and mitigation plans. Through the integration of multidisciplinary static and dynamic data a better understanding of fracture geomechanics in this specific reservoir was reached.

The main building blocks of this integrated study were:

– Thorough QC of the initial BHI interpretations. Re-interpretation had to be performed in multiple occasions to keep interpretation methods and nomenclature uniform. – Analysis of the mechanical behavior of high porosity / low cohesion sandstone reservoirs developed in the field. Evaluation of the likelihood of encountering natural open fractures in such lithologies based on elementary principles of rock mechanics. – Analysis of fluid production history and of dynamic data with regards to the potential origins of early water breakthrough.

Our main results were the following:

– The quantities of natural open fractures originally interpreted from borehole images were significantly overestimated. Electrically resistive traces on BHI were almost systematically interpreted as open fractures in the reservoirs. – The application of elementary rock mechanics principles (based on Mohr-Coulomb laws) does not support the presence of dense networks of natural open fractures in high porosity/low cohesion clastic rocks under the deformation conditions to which the field was exposed. Therefore, the presence of highly connected fracture patterns is unlikely in such rocks. – Dynamic observations indicate that water short circuiting in high permeability matrix is most likely due to the fluid mobility contrasts for heavy oil reservoir supported by relatively strong aquifer (water "fingering" through oil in high permeability matrix).

Based on these results we recommended to reconsider the systematic acquisition of BHI in these reservoir formations as a mitigation for early water breakthrough as well as the systematic isolation of supposedly fractured intervals. Conversely, the improvement of the characterization of the distribution, geometry, and spatial organization of sand bodies and of their connectivity in the reservoir units was suggested. This task is challenging but will be supported by optimizing the utilization of the large amount of already acquired borehole image data. With this aim in mind new images may also be collected in selected wells. A proper understanding of the impact of the detailed reservoir architecture on water versus oil flow through the reservoir is now needed to provide an enhanced mitigation plan to early water breakthroughs. Provisional solutions rely in adapting production parameters such as the rates of oil offtake for the new infill wells. Investing in well and reservoir surveillance (WRS) will also increase the value of the current well stock.