Achieving complete and efficient gravel packing is the primary objective when tasked to install an effective sand control completion. This paper describes the challenges faced during the selection, design, planning and execution phases of the open-hole horizontal gravel pack completions in the "L" field, offshore Republic of Congo. The open hole gravel packs (OHGPs) targeted the most prolific sand to ensure production objectives were achieved with the benefit of a gravel-pack completion. The operator completed two OHGPs with alpha/beta technique for the sand face completion utilizing concentric annular pack screen (CAPS), wire-wrap screens, and a service tool permitting post-gravel pack filter cake clean-up treatments. The accurate selection, modeling, and evaluation of sand control techniques were crucial for completion and production optimization, as well as risk minimization. The service approach had traditionally leveraged upon industry rules of thumb and previous experience, but as the reservoir section length increased and completion configuration became more complex, the error margins tightened and the cost of failure increased such that a more robust design approach was needed. This paper addresses several key factors that must be considered carefully when installing a successful gravel pack completion. The dynamic pressure management becomes critical to maintain the bottom hole pressures within a required range for the successful implementation of the gravel pack. Other factors include designing an optimal screen-wash pipe annulus ratio during alpha-beta wave packing, an optimal alpha wave height, and the use of multiple beta wave packing rates. Results from the preliminary modelling of the gravel pack pumping provided an accurate estimation of the maximum and minimum anticipated pumping rates and pressures. These parameter estimates were generated for the workstring design scenarios and provided guidance for the planned pumping rate adjustment during the gravel packing execution phases. The gravel packed wells achieved pack efficiency greater than 100% based on an estimated 8.5-in. diameter OH. Post completion multi-rate tests concluded potential productivity exceeding the operator's expectations. This paper describes the lessons learned and best practices developed for offshore Congo open-hole gravels packs, which have a very challenging well completion and reservoir scenario. Several proven completion practices are reviewed, with a critical examination of the application of these scenarios for future completion operations in this difficult operating environment. This applied methodology has made significant impact on future field development and increased the production expectations for the asset.
Moho Nord deep offshore field is located 80 kilometers offshore Pointe-Noire in the Republic of the Congo. The wells produce crude from the Albian age reservoir and lithology consists of alternating sequences of carbonates and sandstone layers with high heterogeneity and permeability contrast, including the presence vacuolar layers called "hyperdrains". This paper describes the application of a novel acid system and the methodology successfully applied to effectively acid stimulate the Albian drain. The combination of long perforation intervals with lithology and permeability contrasts, natural fractures, and the potential for asphaltene deposition resulted in adoption of a Modified Carbonate Emulsion Acid (MCEA) fluid system containing a solvent to provide asphaltene deposition prevention. The MCEA stimulation treatments were bullheaded from a stimulation vessel and an engineered diversion process was implemented for effective acid diversion using a combination of mechanical ball sealers and a degradable particle system (DPS). The selection of number of ball sealers and the DPS diverter design depended upon the interpretation of zone permeability profile from the logs, and the final distribution of perforations selected along the drain. A fluid placement simulator indicated low sealing efficiency of the ball sealers would lead to an overstimulation of the highest permeability areas. Subsequent simulations indicated that the DPS would provide better acid coverage with lower skin (S). Results and observations presented indicate that the decision to improve the acid diversion design and combine ball sealers with a DPS diversion technique to improve zonal coverage was validated. During the stimulation treatment execution, the high stimulation treatment efficiency was clearly apparent from the pressure responses to the acid and the diverter system which sealed off perforations and diverted the treatment to other layers with lower permeability. The MCEA also has proven to have self-diverting properties due to its high viscosity and low reaction rate which creates a better coverage of the drain, even with limited pumping rate, allowing live acid penetrating deeper into the formation. The production results reported from the 15 wells stimulation campaign (10 producers, 5 injectors) indicated that the productivity indexes (PI) exceeded expectations and resultant post-stimulation skin values ranged from −2.5 to −4.1. The Moho Nord deep offshore stimulation campaign yielded outstanding production results and showed significant validation for use of the MCEA system and the diversion methodology applied. On the producer wells the use of both chemical and mechanical diversion was valuable, as the DPS proved to complement the Ball Sealers for layers with lower injectivity and also at the high injection rates. High injectivity gain coupled with effective diversion was crucial for enhanced wormholing and good drain coverage.
In this paper we present case studies describing the approach adopted to solve scaling issues in a complex well architecture, an analysis of the scaling root causes, and the construction of a novel execution plan incorporating scale inhibitors, diverting agents with different acid systems to maximize the treatment efficiency. Even when producing at a low water cut fraction, most of the offshore multi-fractured wells in the field experienced scale deposition phenomena because of instability of the calcium ions present in the formation water. When pressure drawdown is applied on the producing wells, a progressive and severe worsening of production performance was observed, and in certain cases this led to a complete obstruction of the well. Previous stimulations executed on the under-performing wells were able to temporarily restore the production. Those treatments were performed using a conventional HCl acid system with coil tubing and these yielded positive results initially, but performance progressively decreased after a few months. For this reason, it was a priority to analyze the root cause of the deposition and define an improved method to extend the effectiveness of the intervention. Scale tendency analysis of the formation water highlighted the instability and predicted calcium carbonate presence at the reservoirs’ pressure and temperature range. Based on the evaluation of Saturation Index it was determined that calcite build-up can occur at any point in the production system. This was confirmed by field evidence, with scale deposit samples recovered at the choke, surface line and along the completion tubing. A nitrified organic acid blend was applied to invade deeply into the fracture body, together with a liquid scale inhibitor squeeze treatment that was designed to prevent further re-depositions in the short-term. A diversion technology was implemented to treat the multi-fractured horizontal wells in efficient manner by rig-less bullheading. Furthermore, due to unavailability of a rig in place, efforts were made to solve the different challenges to operate in rig-less mode: a lack of space on the production platform deck prevented any pumping intervention, and the well restart and clean up was executed directly in a high-pressure sea line. This alternative approach, with novel technologies for diversion and scale inhibition, yielded excellent well responses to the placement of the acid mixtures, which were designed to dissolve the carbonate scales with minimum impact on the sandstone formation, completion equipment, and production facilities. The selected solid diverting agent self-degraded by hydrolysis once in contact with water base fluids in the high temperature environment. This diverter was able to effectively distribute the acid treatment into each of the fractures: the particle size distribution was designed to efficiently bridge on the proppant pack in the fractures. The well start-up production rates confirmed the major benefits resulting from this approach: a higher Productivity Index was estimated on all the applications when compared to past conventional stimulations. Moreover, the use of a scale inhibitor extended the post-stimulation well life from few weeks, up to several months or years and therefore reduced the frequency of future well interventions. This novel alternative approach resulted in a more cost-effective well intervention solution and addressed the challenges of an intense offshore rig-less stimulation campaign in the field.
Gravel-packed wells in the "C" field located in offshore Angola are prone to damage by a variety of factors including scales, fines migration, paraffin and asphaltene deposition resulting in skin values of 45-95. This paper focuses on the approach used for 2 subsea open hole gravel packed wells located within "C" field. Rigless subsea stimulations in approximately 470 m of water using an intervention vessel with the downline deployed via the vessel moonpool. Additionally, a stimulation vessel was utilized to provide pumping and fluid capacity without disturbing the primary intervention operations. This paper documents the efforts made to restore the wells forecasted production by bullheading the acid stimulation treatment from the stimulation vessel through the open-water hydraulic access system installed on the intervention vessel. Well history attributes the impairment to fines migration accumulation and scale and paraffins deposition. The proposed stimulation fluids were designed to treat as many damage mechanisms as possible during a single intervention. The basis for design incorporated a primary solvent pre-flush to clean possible paraffin and asphaltene deposition as well as prepare the reservoir and proppant pack for further stimulation fluids by stripping away hydrocarbon residue. The preflush was followed by a second treatment fluid consisting of HCl acid to remove any carbonate-based damage. The final treatment fluid utilizing a combination of HCl acid and hydrofluoric acid (HF) was specifically designed to remove fines contained in the gravel pack and screens. Injectivity tests were performed to evaluate the reservoir prior to and after the acid treatment as well as to help understand the damage mechanism. Based on the bottomhole pressure response during acid-treatment stages, measurable improvements were evident on both wells, which supports the pre-treatment damage diagnosis. The efficient and cost-effective execution of the treatment campaign, combined with the conclusive post-stimulation production data, confirms the effectiveness of open-water hydraulic access by utilizing an intervention vessel and a stimulation vessel, allowing to provide pumping and fluid capacity without disturbing the primary intervention operations on complex subsea wells. Post-stimulation results after the successful removal of wellbore scale and formation damage in the two subsea wells showed an average increase in oil production of 60%. Skin damage was reduced by 66% on Well A and a complete removal of skin on Well B. The results confirm the effectiveness of cost-driven acid stimulations on complex subsea wells without the use of a drilling rig as well as demonstrating the ability to address multiple damage mechanisms from a single intervention.
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