Acid fracturing or matrix acidizing are often required for increased hydrocarbon production and long-term well deliverability from the massive gas Khuff carbonate reservoir in Saudi Arabia. Openhole multistage stimulation (OHMSS) technologies have demonstrated superior performance in maximizing reservoir contact and productivity through better distribution of acid across the formation matrix, full interval matrix contribution and efficient propagation of fracture networks to bypass formation damage and optimize near wellbore (NWB) conductivity.
The Khuff structure is a late Permian age heterogeneous carbonate sequence that underlies the massive Ghawar field in Eastern Province of Saudi Arabia. The Khuff reservoir is subdivided into four separate intervals (A through D), with production coming mainly from the B and C intervals. Since its initial appraisal in the late 1970s, the majority of Khuff development activities have been focused in the relatively more prolific Khuff-C formation, where Coiled Tubing acid wash and single stage acid treatments were releatedly performed and evaluated. Multistage acid fracturing have been implemented in Saudi Arabia Khuff-C development in the last five years and results were carefully evaluated for each trial, based on which it is now the predominant Khuff-C stimulation technique.
Up until the middle of 2011, the vast majority of Saudi Arabia's horizontal Khuff carbonate gas wells were drilled along the direction of maximum horizontal in situ stress (σmax). This was primarily to enhance wellbore stability and achieve best possible penetration rates. Early multistage fracture treatments in the Khuff generated mostly longitudinal fractures propagating parallel to the wellbore, in the direction of σmax. Since then, a holistic approach toward the application of OHMSS technology for tight reservoir development was adopted.
The complex workflow of this approach called, among other requirements, for changing the lateral section placement strategy and planning the horizontal section to be drilled along the minimum horizontal in situ stress direction (σmin) as opposed to the previous mode of planning along the σmax. Accordingly, understanding the reservoir stress profile, orientation of the horizontal wellbore with respect to the dominant horizontal stress component and calibrating the stress profiles against actual openhole (OH) logs became the most important highlights of the new workflow. Radical improvement of stage integrity, multiple fracture signatures and enhanced well productivity were among the most important results achieved.
Still, an innovative approach was required to address the more prolific Khuff-C formation where efficient matrix acidizing was sought as an alternative to acid fracturing in wells that could only be drilled in σmax direction. Therefore, a purpose-built OHMSS technology system that was developed around the idea of distributed limited entry for placement of matrix acidizing treatments was identified and carefully evaluated.
This paper will present the details of the successful application of this new limited entry, multiple injection technology for optimized matrix acidizing of carbonate horizontal wells, including candidate selection, system design, functionality, operation and ultimate production profiling.