Creating Multiple Fractures in Cemented Horizontal Laterals: An Efficient Way to Improve Productivity of Tight Carbonate Formations in Saudi Arabia
Authors: Rahim Zillur , Al-Anazi Hamoud , Kanan Adnan , Kayumov Rifat Eduardovich 
Published: March 2015

Abstract
During the last decade, significant efforts in developing gas reserves from relatively tight carbonate formations have been made in the southern part of Saudi Arabia. An important role in this effort was played by the multistage acid fracturing technique as it enables maximizing reservoir contact by creating multiple fractures along the wellbore. Lessons learned from many such treatments highlighted the following challenges related to openhole (OH) multistage fracturing (MSF) completions:Communication between stagesBottomhole treating pressure (BHTP) dropping below fracture gradientLimited reservoir contact due to only one fracture propagating between OH packers
A cased hole plug and perforation (P&P) completion method was proposed to effectively solve the challenges associated with OH completions. Cement behind the liner provides an excellent seal between stages that cannot be achieved with either mechanical or swellable OH packers. A short perforated interval will create an orifice pressure drop at the entrance of the fracture during the treatment, which enables keeping the BHTP above the minimum horizontal in-situ stress (σmin), thereby keeping the fracture open. With a proper geomechanical modeling, few perforation clusters can be optimally placed in one interval that can all be treated by a single stage, with individual fractures growing from each cluster. This will maximize reservoir contact.
P&P completion is a proven method for proppant fracturing of horizontal wells in unconventional reservoirs, but its implementation with acid fracturing in carbonates is not well-described in the literature. A reservoir simulation study was performed to prove the benefits of P&P completions. The study shows that P&P outperforms the OH MSF completion for a wide range of permeability values. Additional cumulative production varies from just 2% for good quality reservoir with permeability of 1 md up to 26% for tight formation with 0.01 md permeability.
Three horizontal wells were selected for stimulation treatments with P&P completion in the southern region of Saudi Arabia. Reservoir contact area was significantly increased by placing 12 fracture stages with a total of 32 perforated clusters in these three wells. Communication between stages was eliminated and BHTP stayed high to continue fracture propagation.

Introduction and Problem Identification
The Khuff carbonate formation was discovered in the eastern part of Saudi Arabia beneath the Ghawar giant oil field in late 1970s. This formation is a late Permian heterogeneous carbonate sequence that is subdivided into four separate intervals. The gas-bearing layers have been found mostly in tight dolomitic and the more prolific calcite. The reservoir is a high-pressure/high-temperature (HP/HT) carbonate that exhibits extensive heterogeneity in stress, reservoir quality, bottomhole pressure and temperature, and reservoir fluids throughout the field. To date, the majority of Khuff development activity has been focused in the more prolific areas. With growing domestic gas demand in Saudi Arabia, significant efforts are underway to develop the tighter areas with increasing number of wells and application of novel technologies.