The trend in the oil and gas industry is to drill wells at greater water depths even though operational difficulties can increase and rigs capable of drilling such wells have a high day rate. During the exploration campaign in the Santos Basin (Figure 1), the operator drilled about 20 wells in ultradeep water. The reservoir included multiple zones of interest and hydraulic isolation that allowed selectivity between the zones. Additionally, the presalt environment demanded cementing of the production casing to help prevent casing deformation/ collapse. Therefore, cement integrity was important to the success of the proposed plan.
FIGURE 1. An operator in the Santos Basin drilled about 20 wells in ultradeep water. (Source: Halliburton)
In the Brazilian presalt environment (Figure 2), the reservoir consists of a series of heterogeneous hard microbialite carbonate layers up to 900 m (2,953 ft) thick located below a salt layer 2,000 m (6,512 ft) thick that serves as a reservoir seal. The narrow margin between fracture gradient and pore pressure inherent of deep water, presalt and the depleted zones in mature assets generates low equivalent circulating density (ECD) requirements. Uncontrolled ECDs can increase the potential risks of fracturing pressure-sensitive formations and induce lost circulation that can increase the potential risks of wellbore instability, packoffs, stuck pipe, well-control issues, formation damage and even the inability to complete the well. The problems are particularly magnified when circulating mud, running casing or liners, and cementing in high-angle extended-reach and horizontal well geometries.
FIGURE 2. The Brazilian presalt environment presents cementing challenges. (Source: Halliburton)
The loss of fluid into formation can be a significant complication while drilling and cementing wells, resulting in considerable nonproductive time on the rig and additional costs. Managing severe lost circulation while cementing can be challenging in highly permeable zones and naturally fractured formations. A novel solution consists of an engineered composite lost-circulation material (LCM) solution with particle-size distribution to potentially manage severe loss-circulation situations in naturally fractured reservoir formations.
The lost circulation issue should be addressed before cementing operations begin since there are several conventional methods or strategies available to help prevent or mitigate losses before cementing such as lowering the weight of the fluid, incorporating LCM into the drilling fluid and pumping gelling agents that can bridge off loss zones. However, conventional methods are not always effective, and the available solutions to minimize lost circulation during cementing are limited (granular or fibrous materials incorporated in the spacer and cement slurry, reactive spacer, increasing hole excess, thixotropic slurries, foamed cements, etc.).
Various types of LCMs have been applied throughout the years using a variety of particle sizes and shapes. Many competent materials are not allowed during deepwater operations, so it was necessary to develop a material that qualified for use in this area. At relatively low concentrations, round and heavy lost-circulation particulate material is difficult to suspend in a spacer pit. Additionally, heavy materials such as calcium carbonate tend to increase the density of otherwise light spacer fluid.
Particle shapes that deviate significantly from the spheres are easier to suspend in fluid, and they also bridge off better across fracture openings. Fibers are known to bridge off effectively but hold little differential pressure at large fracture widths. Resilient materials are known to produce a long-lasting seal in fractures when the pressure varies, but in large fracture widths their bridging capability is limited. Flaky materials bridge well if they do not orient sideways in the fracture, but many of these materials only hold a low differential pressure.
The combination of high-performance components with different characteristics in the LCM spacer fluid material generates a versatile high-pressure rated effective LCM with minimal leak-through potential. These components act synergistically to mitigate lost circulation by bridging tight slots effectively at moderate concentrations. Once formed, the bridge can easily withstand a 1,000-psi differential pressure.
Spacer fluid enhanced with the addition of the LCM is designed to overcome lost circulation while preparing the wellbore to receive cement. It is an environmentally acceptable blend of carefully selected materials, including coarse and tough LCM, fibers and medium-sized resilient angular material. This fluid system helps mitigate losses when cementing across weak, unconsolidated or fractured formations. The LCM spacer fluid allows customized viscosity, and the particles remain in suspension with minimal potential risk of sedimentation. Additionally, it can accommodate weighting additives to optimize the fluid density.
A well was drilled with a 12¼ in. bit and a 9.8-ppg to 10.2-ppg synthetic-based mud across the reservoir formation to produce oil from the carbonate reservoirs. The open hole was logged, exhibiting an average caliper measurement of 12.45 in. With the production casing/ liner at the bottom, the fluid was conditioned for 3 to 4 hours, and a lost circulation of 300 bbl/hr to 500 bbl/hr at 10 bbl/min was observed.
An LCM spacer fluid was proposed by the subject operating company, a solution that was proved to mitigate loss of circulation across the carbonate section. To apply this advanced loss-circulation solution to presalt fields, the operator and the subject operating company worked together, evaluating previous field applications of the LCM spacer fluid package and conducting rigorous laboratory tests of the LCM spacer fluid in spacers.
To assess the potential risk of bridging across the narrow annulus, this spacer was tested, simulating a size of 5/32 in. (4 mm) as a worst case and the smallest the operating company could recommend at different concentrations (10 parts per billion to 25 parts per billion). According to the test results, the formulation was better suited to the necessary conditions. Single slurry was designed to cover the entire annular span of the intermediate casing. The cementing operation involved adding 15 ppg of the LCM spacer fluid package in the spacer on the fly.
Cement was successfully placed across the reservoir zone in a single cementing stage, and circulation with losses between 300 bbl/hr and 500 bbl/hr were reported during the cement operation. When spacer containing the LCM spacer fluid reached the annulus, the return was regained and maintained until the cementing operation was complete. Cement-bond log evaluations showed good bonding to pipe and formation, indicating good zonal isolation that met regulatory requirements throughout the entire pay zone. Using the LCM spacer fluid package helped the operator avoid costs previously experienced because of lost circulation events across the carbonate.
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