Total operates an offshore oil field approximately 60 Km West of Pointe-Noire, Republic of Congo. To date, the Albian reservoir has contributed most of the oil produced. However, a significant part of the oil in place is in the Cenomanian, a low permeability sandstone formation with poor quality. As of early 2007, only two of the initial 50 wells in the field had been completed in the Cenomanian. It was desired to evaluate the potential of hydraulic fracturing stimulation treatments to more fully comprehend the production capability of this reservoir. In early 2007, a third well was drilled and completed in this structure, penetrating the Cenomanian at a 60 degree inclination for this purpose. Proppant fracturing from a wellbore with a 60 degree inclination in a hard-rock formation has always been difficult. Also adding another level of uncertainty was the fact that the Cenomanian has never been fracture stimulated. This paper will detail the planning process from both the operator and the service provider perspectives with specific well control issues related to an over-pressured reservoir. Review of the well conditions and operational constraints indicated the best approach would be a multi-stage pinpoint stimulation method incorporating hydrajet perforating and sand plug isolations using a stimulation vessel especially designed for such work. Additionally, a comprehensive review of the actual three-stage stimulation treatments will detail the perforating, fracturing, and sand plug isolation after each stage. With BH memory gauges just below the jetting tool assembly, post-frac reviews of downhole pressure data is compared with the live annulus data used for real time decisions during the stimulation stages. Post-frac well cleanup and early production data will be included. Background Of the initial 50 wells in the field, only two producers were completed in the Cenomanian instead of the Albian reservoir. Although this reservoir constitutes a major part of the oil reserves of the field, its low permeability (10mD) and poor formation quality (argillaceous siltstones) always result in a low production index in perforated cased hole configuration. To show that a field development on such a reservoir could be economic, it was needed to prove that effective fracturing operations could be performed on the third well, drilled early in 2007, in this structure. No similar operation had been undertaken in West Africa on a hard low perm rock where the core samples appear to come from a well in Texas, instead of offshore West Africa. Furthermore, a fines stabilizer chemical agent was pumped to provide a treatment solution to lock the migrating fines in place to minimize the potential damage to the formation and maintaining the production flow rates of the well. This offshore field is currently operated from two primary platforms set in about 140 m of water. Most of the oil production has been from the 48 wells completed in the Albian reservoir. This reservoir, below the Cenomanian, is a moderate permeability carbonate formation with a varied degree of natural fracturing present. It is typically completed using a large volume HCl acid-frac stimulation after perforating. These wells account for more than 97% of the pre-2007 production of the field What enhances the value of the Cenomanian formation as a producing reservoir (while increasing the complexity of drilling and completion operations) is that this reservoir is significantly overpressured. The reservoir pressure in these zones required the use of a 1.40 sg completion fluid (mixed CaCl2 / CaBr2 brine) to maintain 8 bar over-pressure above reservoir pressure.

Total E&P Angola operates the deep offshore ROSA field on Block 17. On the fifth development well in the field. Two zones in the Oligocene sandy layers are transversed at a well deviation of 48° and required the implementation of the stacked frac-pack technique. The perforated length of zones was approximately 118 ft and 213 ft. Multiple fracture initiation and early screen out were expected, requiring consideration for an enhanced frac-packing method. In order to facilitate placement reliability, a concentric annular packing system was chosen to limit slurry bridging during placement in combination with an aggressive pumping design (25 bbl/min for the lower zone and 32 bbl/min for the upper zone). This paper will discuss the system used for successful frac packing and how it provided alternate paths for the slurry flow when a bridging occurred while keeping the same high flow rate. Data collected on the two zones during treatment have shown that early annular bridging had occurred during slurry placement, but the system had successfully created a high-velocity flow path with minimal pressure drop to bypass the bridge. Once the bridge was bypassed, the full flow area was again used, minimizing turbulence and potential for damaging screens. The paper will describe:The target zonesThe concentric annular packing system designIntegration of the concentric system in the dual stacked frac packs architectureHole pattern design and velocity calculations in shroud orificesThe service tool design for stack frac-packsPumping treatment analysisObservations gathered during the jobHow technique and job-treatment design versus use of a regular frac-pack system or other alternate-path systems was justified. Although alternate path technique has been used in frac-packing more than 30 times globally, the stack frac-pack performed on this case history well (WELL-5) holds the record for being the longest treated length, steepest deviation, and highest pumping rate ever attempted thanks to the concentric annular packing system originally designed for the horizontal gravel pack completions. High completion efficiency was achieved. Productivity data are provided (skin, PLT, etc). Introduction The Rosa field was discovered in January 1998 and was appraised with two additional wells at the beginning of 1999 and at the end of 2000. The field is located in Block 17, offshore Angola, 210 km northwest of Luanda, in an average water depth of 1,400 m (Fig. 1). Rosa field is one of several fields in this prolific block. Under a production sharing agreement with Sociedade Nacional de CombustÍveis de Angola (SONANGOL), the Angolan state oil company, partners in the field include Total E&P Angola (40%, operator), Esso Exploration Angola (Block 17) Ltd (20%), BP Exploration (Angola) Ltd (16.7%), Statoil Angola Block 17 A.S (13.3%), and Norsk Hydro Dezassete A.S. (10%).