Delayed Action Micro-Emulsion Breaker and Its Applications to Improved Completions Operations – An NNPC/FEPDC JV Approach During Completions Operations

Abstract

In horizontal open hole wells, the formation of filter cake while drilling the open hole section of the well is desirable. This filter cake serves the purpose of forming a semi-impervious layer around the reservoir drain-hole. This layer helps reduce losses considering the overbalance required for well control during drilling. It also serves as an additional structural support to keep the open hole stable when the drilling bottom hole assembly (BHA) is pulled out of hole and the screens and lower completions accessories are being run in hole. However, when thewell is put into production, the filter cake becomes a contributor to skin and poor reservoir productivity. It is therefore required to get rid of the filter cake after running the screens and the lower completion. Having procured and prepared the sand screens for deployment after drilling the open hole section, it is important that they are run to the bottom successfully with minimal damage and plugging. Usually, the open hole section of the horizontal well is drilled with specially formulated drill-in-fluids (DIF). Since this section is drilled in over balanced mode, the exerted pressure keeps the hole open so that the sand screen can be run successfully. The DIF replaces the drilling mud used to drill the earlier hole section(s) but in addition to providing well control via overbalance and transporting cuttings from the hole to surface, it also minimizes invasion damage to the reservoir pay zone. A commonly used weighing material when densities up to 11.5ppg are required for well control is calcium carbonate (CaCO3). When densities above 11.5ppg are required (for deeper, abnormally pressured reservoirs), it becomes necessary to weight up the mud with a heavier material, usually barite + CaCO3. During the drilling process, this overbalance pressure exerted on the reservoir forces the CaCO3 out of the DIF solution and it forms a semi-impervious filter cake on the sand face of the reservoir. This desirable filter cake helps minimize excessive fluid losses into the reservoir hence limiting invasion and damage. It also contributes to the structural integrity of the open hole, keeping it stable prior to running of the screens. Depending on the weighting material used in the drilling of the reservoir drain-hole, the micro-emulsion breaker (MEB) can be designed to break down the filter cake and any undisolvedparticulates can be mobilized and water-wetted and can be then flowed during production or injection. The challenge is that depending on the lower completion configuration, it may take some time to get the wash pipe and work string out of the lower completion and close the formation isolation device. In some cases, it is possible for the formation isolation device to fail. If the Micro-emulsion Blend (MEB) is quick acting, any of these scenarios can lead to uncontrollable losses and serious difficulties in continuing the completion operation. This elucidates the need for a delayed acting MEB treatment. Lab tests and analysis involving the exact DIF /filter cake and various compositions of the MEB at downhole conditions to arrive at the required delay in action. It is critical to ensure that the delayed action does not result in reduced efficacy of the treatment. Hence, the MEB is not diluted for slow action but rather it is engineered combinatorially with a retarder and downhole mild acid generating microemulsion chemistry that gradually generates the necessary mild acid that will slowly dissolve the bridging materials (eg. calcium carbonate) in the mud withtime and allows the full strength of the MEB to take effect after the stipulated delay period. This paper will focus on the lab analysis and iterations to arrive at an optimal MEB blend.