Many West Africa Offshore Fields are maturing and operators are completing secondary targets in their wells to maintain the economic operation of their valuable assets. However, off-shore environment makes the capital expenditure associated to this kind of interventions of critical importance. It follows that the selection of the right and most remunerative activities is crucial. In the Kitina Field, offshore Congo, deeper sands have been produced to economic depletion and reservoir studies allowed the determination of alternative production intervals for production maintenance. Large quantities of reserves can be found in low permeability, consolidated, formations as well as in very deep and remote culminations. During the first semester of 2007, the Kitina field production increased of 160% reaching a production level lost since early 2004. This was achieved with a variegate set of actions on different reservoirs:infilling the Kitina South culmination with the long reach and ultra deep well KTM-SM5,a massive multistage hydraulic fracturing campaign carried out on the three wells draining the low permeability 3A reservoir and 3rd) with the sweep optimization of the reservoir 1A. Eight propped hydraulic fractures were placed in three re-completed, cased-hole wells with very significant production improvements. These represented the first applications in Congo of different technologies opening a wide range of further applications in similar environments. The paper describes the 2007 and 2008 Kitina rejuvenation campaign with an eye to all the disciplines involved, from reservoir engineering and modeling, to operation geology, drilling and completion, production. The papers focuses with more detail on the successful multi-stage hydraulic fracture campaign from the preliminary design and production forecast pre-job to the reservoir model history match and forecast phase post-job. Interesting reservoir engineering overviews of the future development of the field via improved and enhanced oil recovery techniques are also presented. Introduction Existing oil and gas fields are maturing and new finds are more complex to discover and produce. In today's oilfields portfolio, mature reservoirs production maintenance and increase represent the biggest challenge to face over the next decades to meet the continuously increasing demand for hydrocarbons. Technology research, development and innovation have been the recent answer to sustain the world's oil and gas production and will continue to be so. However, new developments in technology need professionals who are taking the risk of testing them keeping in mind that the failure can be sometimes only a temporary and/or necessary stop towards the success. Mature fields can represent the "working ground" and "technological gyms" where to test new techniques with the final aim of accelerating and increasing reserves. Mature fields have to be seen today as opportunities for improvement rather than declining assets. Accurate candidate selection, optimized treatment design, sound reservoir modeling of production forecast represent crucial and interdependent factors for successful economic evaluations.

This paper describes the successful combination of different stimulation techniques during the development phase of an offshore field in Congo. A total of four wells were drilled and completed during 2008-10. The target reservoir, the Sendji carbonates, has a total thickness of 150-170 vertical meters with an interlayer of ten meters. It is composed of silty-shaly dolomitic and quarzitic sandstone with interbeds of shale and sand grading to sandstone and silt with dolomitic cement characterized by very poor petrophysical properties. Laboratory testing with cuttings showed that formation is more than eighty percent soluble with fifteen percent hydrochloric acid. The typical well has an average azimuth and inclination of 60 and 350 degrees respectively, in the target reservoir. A good quality cementation job of the casing was required in order to ensure isolation from the aquifer. The well completions are both open and cased hole multistage fracturing completion systems. Selected zones of varying lengths were hydraulically fractured by using proppant / seawater-based borate crosslinker fluid or by using twenty percent delayed, viscoelastic and straight acid. Two wells were stimulated by means of proppant fracturing while the other two by acid fracturing. Due to upper / lower water zones with no bounding beds, it was important to define formation mechanical properties for fracturing designs in order to avoid fracturing into water bearing layers. A Mechanical Earth Model (MEM) was created by using sonic anisotropy measurements, modular dynamic test results, and other common electrical logging data together with drilling parameters. The detailed study of design methodology, different stimulation fluids and operation sequences are described. Particular attention will be given to the comparison of well responses to multi-stage proppant and acid fracturing techniques.

Brownfield field have been defined as mature field in a state of declining production or reaching the end of their productive lives. Nevertheless these fields provide more than 50% of world's production as well as world's reserves and so it will be for next 20 years. For that reason revitalization of mature fields has to be a "must". The Kitina Field, offshore from Pointe Noire, Congo, is one such field. Deeper sands have been produced to economic depletion and, in order to maintain the economic operation of their valuable assets, alternative production intervals have been opened to production. Most of these low permeability reservoirs can produce at economic rates only with the application of hydraulic fracturing treatments in a multi-fractured well scenario. Accurate candidate selection, optimized treatment design, sound reservoir modeling of production forecast represent crucial and interdependent factors for successful economic evaluations. This paper describes the massive hydraulic fracturing campaign carried out between April and June 2007 on the Kitina 3A reservoir, offshore Congo. Eight hydraulic propped fractures were placed in three re-completed, cased-hole highly deviated wells with very encouraging production increases (stabilized production increase ranging from 2 to 3 times). The transverse multi-fracturing technique was adopted. This technique utilizes a series of packers and frac-ports that are sequentially shifted allowing continuous placement of more than one hydraulic propped fracture without shutting down the pumping equipment. All the aspects from candidate selection, pre-job design, on-site operations and post job evaluations with particular focus on the operational challenges encountered are described. Recommendations for the future applications of this stimulation technique are proposed. Furthermore, the challenging theme of forecasting mid-long term production profiles for horizontal, slanted and vertical multi-fractured wells is tackled. Different analytical and numerical models, approaching the task with various methodologies, were applied and tested to define the production profiles of the three candidate wells, which differ in terms of geometry and reservoir properties. On the base of the experience gathered on such wells, some general guidelines were drawn for a wider application. Introduction The Kitina Marine offshore field was discovered in 1991 and production start up was in December 1997. Originally, the field development considered the three deeper intervals that were developed via a peripheral water injection scheme and a crestal gas injection displacement process. After a quite significant initial rate (around 50,000 BOPD), the field declined quite rapidly and it was necessary the installation of gas lift valves on all wells. In 2005, aiming to reduce the inevitable rate decline, a new level was opened to production via the recompletion of three deeper wells into the shallower and low permeability 3A reservoir [Ref.1[. This interval is characterized by the presence of thin silt and shale layers that decrease the vertical permeability strongly. The 3A Sandstone reservoir was initially produced in natural flow between the end of 2005 and august 2006, although the three wells were completed with gas lift mandrels (Fig. 7, Fig. 8, Fig. 9). Due to the low permeability (2–7 mD), after first year of production, level 3A in Kitina field had progressively declining production heading to a marginal-economic scenario. At the beginning of 2007 the production was:KTM W6 ST - 160 BOPDKTM 107 - 130 BOPDKTM 111 - 300 BOPD

Production of heavy oil with an artificial lift system is a puzzling problem due the hydrodynamic conditions prevailing inside and surrounding the pumping system. The aim of this paper is to present an application on ESP PCP of a system that has an impact on the oil viscosity surrounding the ESP PCP unit and as a consequence improving the production performance, the ESP PCP pump efficiency and run-life. The theory is based on the impact of convection currents on the diffusion of the temperature in high viscous oil inside a pipe. A case history will be presented on the ZAM-408ML, a TAML6 multilateral well producing from level B of the Zatchi field, offshore Congo. Introduction Zatchi Marine is an offshore field located about 25 km from the Congolese coast (fig.1). The field, developed in 1980 presents 5 oil bearing levels named A, B, C, D and E. Level A has currently no economical value since it is gas bearing. While the oil bearing levels C, D, E are producing from several years, level B, after very short episodes of production in the past was put in production in 2007 via the multilateral well ZAM-408ML. The level B, object of the paper, is a complex heavy oil reservoir with a primary gas cap and the presence of a bottom aquifer. The oil has a very high density (15°API) and is very viscous (13000 cP @ 34.5°C) probably due to the low reservoir temperature (about 35°C). This level is composed of sand, shaly sand, dolomite and shale (fig.2). The average reservoir permeability is greater than 1Darcy. Three wells have been drilled in the B level since 1991: the first one, ZAM-116 (vertical well with conventional completion), produced a maximum of 130 bopd (lifted by a sucker rod pump with a surface motor); thereafter, the ZAM-406 (horizontal well) produced at the beginning 440 bopd but the oil rate dropped rapidly due to a severe gas coning phenomena; the last one, ZAM-111ST (horinzontal well), peaked briefly up to 285 bopd. This brief overview depicts the reservoir B production complexities which in summary:the heaviness and the viscosity of the oil,The severe problems of gas coning and cresting which reduced drastically the progressive cavity pump efficiency. The multilateral well ZAM-408ML represents the forth well which is attempting the production of such reservoir. The well architecture and trajectory was designed to optimize reservoir recovery, productivity index while reducing the drawdown at the wellbore to avoid gas coning phenomena. In parallel, it has been decided to develop an innovative system to improve the ESPCP performances by increasing and homogenising the temperature distribution around the artificial system planned to be installed. The original scope was not to improve the well productivity, but only to have a better handling of the flow at the pump interface and maximizing the pump efficiency.

This paper presents a case history of a technique utilized in a loose sand fontation to keep constant pressure underbalance during tubing fill-up after TCP shooting. The method allows a good evaluation of the formation characteristics scon after the TCP shooting, providing an early knowledge of the formation productivity before the final completion with gravel-pack and Electric Submersible Pumps. Moreover, it gives the means for a correct pump design and the evaluation of the gravel pack efficiency. This technique developed by Agip Recherches Congo was applied to shallow reservoirs consisting of loose sands filled with rather viscous oil. All the wells are completed with gravel packs and electric submersible pumps (ESP). Prior to shooting the tubing is filled with nitrogen and the pressure is discharged to have desired under-balance. To keep underbalance constant during tubing fill up, WPH is discharged to decrease backpressure. The fact that a practically constant pressure underbalance occurs during perforation clean up allows a reliable interpretation of the bottom hole pressure transient.