In the Mexico marine region, gas breakthrough is common in naturally fractured carbonate oil reservoirs. Increasing the gas production reduces crude oil production, and eventually the wells become uneconomic and are shut-in in spite of the remaining recoverable reserves. A typical example is the Akal field, a large fractured 300-to 1000-m thick carbonate reservoir whose permeability varies between 0.3 and 5 darcy. The gas-oil contact zone moves by as much as 8 m/month as the natural gas and nitrogen gas from gas injection moves through the natural fractures and invades the oil zone. This condition results in production decline, reservoir pressure decrease, and oil remaining in the matrix.Efforts to selectively shutoff the gas have been unsuccessful due to the low-reservoir pressure and high-permeability contrast. When pumping water-based fluids, the increased hydrostatic pressure causes the treating fluid to travel down the natural fractures and away from the gas cap. This condition led to abandoning the gas-invaded intervals and recompleting lower in the reservoir, leaving some recoverable reserves.To selectively shutoff gas entry in fractured reservoirs, a stable foam-delayed crosslinked fluid was proposed for use by service company. The fluid with a high-foam quality (FQ) and low density rides over the crude and into the natural fractures/fissures, communicating with the gas cap. Once set, the fluid creates an impermeable seal with high-extrusion resistance.The stable foamed fluid has been successfully used to selectively shutoff unwanted gas production in wells that have been, in some cases, shut-in for several years due to excessively high gas/oil ratios (GOR). Following the treatment, the oil production was restored to the same level as prior to the gas breakthrough.The success of the initial campaign demonstrated that it is possible to restore the production levels of shut-in wells and recover otherwise lost reserves. This result has a very significant positive impact on the economics of operating the field. The current plan is to extend the use of the technique to other fields.

The Cantarell field is located offshore in the Bay of Campeche, in approximately 50 m of water depth (Fig. 1). The two main productive intervals are the Upper Cretaceous and Jurassic Kimmeridgian. The Cretaceous is highly fractured with well developed, vugular secondary porosity. The Jurassic is also fractured with oomoldic porosity. Both contain sour hydrocarbons. Production from Cantarell began in the early 1980s and today the pore pressure gradient is down to a +/− 0.37 gr/cc equivalent. The productive interval is typically drilled using a 0.90 gr/cc emulsion mud and shortly after penetrating the fractured reservoir, total and uncontrollable loss circulation is experienced. As a result, the cost of drilling in the field has increased considerably due to the high cost of the oil-in-water mud losses and time spent either transporting, generating or waiting on weather to offload more mud to continue drilling. Petróleos Mexicanos (PEMEX) in cooperation with Quantum Reservoir Impact (QRI) reevaluated the current drilling philosophy and recommended the application of a series of steps aimed at solving the current problems. The results obtained have been positive resulting in PEMEX now implementing such steps as part of its drilling philosophy where applicable on a regular basis. This paper will describe the current drilling environment and the techniques implemented in the first successful mud cap application in the Cantarell field. It will also discuss some of the lessons learned as well as the new improved designs being implemented and will conclude with a summary of the benefits obtained and the plans for the future.

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In the tight carbonate reservoir located in onshore Mexico, fault damage zones and associated fracture networks represent major permeability structure and greatly affect fluid flow. In this study, we present a method for characterizing and modeling fault damage zones and fracture networks. The main objectives are to use fault damage zone and fracture network models to predict fluid-flow pathways in the reservoir and to understand their impact on well production performance. Appropriate geometric attributes are used for unsupervised seismic facies classification with the aim of delineating fault damage zones. The utility of the facies approach lies in its capacity for classifying the fault system with distinct classes based on the degree of fracturing. The three-dimensional fault damage zones are extracted and modeled from the seismic facies volume. A natural fracture network model in the target formation is deterministically built by meshing fracture lineaments that are automatically tracked along depth slices of the edge-enhanced curvature attribute. The fracture network model provides fracture characters in terms of fracture orientation, length, density and connectivity. Local fracture network models at individual wells can be built using fracture connectivity analysis technique. The fault damage zone model delineates fault zone architecture and spatial variability. Our results show that the main fracture sets parallel or subparallel to the fault zone strikes regionally. Fractures distribute surrounding fault damage zones with enhanced fracture density. A good correlation between the fault damage zones and the lost circulation is observed. Based on the production data, fault damage zones can act as fluid conduits. In some instances, fault damage zones can create flow barriers. Conditioned with well data, the conductivity of the fault damage zones and related local fracture networks is calibrated. Using the local fracture network model, the fluid flow pathways and fracture vertical connectivity are determined. With borehole breakouts and drilling induced fractures, the identified maximum horizontal stress orientation is in the NNE direction. Our results show that the conductive properties of fault damage zones and fractures could be affected by the stress state and in situ stress orientations. This explains the fact that the wells located in the fault damage zones and fractures with the orientations preferable for extension tendency or slip tendency have high productivity. The continuous fault damage zone model and the discrete fracture network model can be used to capture 3-D complexity of fluid flow in fault damage zones, improve the prediction of flow pathways and help the lateral or horizontal well design.