Formation damage in horizontal wells, often open hole completed, is a critical point for oil fields developed in deep offshore where acceptable development costs are based upon a limited number of highly productive wells. This paper describes a simplified numerical approach to model filter cakes removal by natural cleanup when the well is put under a drawdown pressure difference. A flow simulator taking into account the near well permeability variation has been developed using cylindrical grid with very small gridblocks around the well. Laboratory data obtained on sandstones damaged with an oil-based mud have been used as input data to model filter cakes removal by natural cleanup. The purpose of this modelling is to optimize the applied drawdown pressure to study the variation of well productivities as a function of the pressure drawdown to get the best possible well performance preventing sand production problems in poorly consolidated formations.
Deep offshore reservoirs are usually developed in very severe conditions, and so, their development costs are pretty high. The profitability of such developments requires a production scheme involving a limited number of wells having very high productivities and acceptable life-spans of several years without any dramatic interruption. To reach these objectives, horizontal well of great length are generally considered. These wells are mostly completed open-hole (non cemented), avoiding any possibility of efficient remediation or stimulation in the case of an important damage would be created in the vicinity of the wellbore. Therefore, the clean up of these wells when put initially into production must resorb the greatest part of the damage generated during drilling and completion operations. An additional complication comes from the fact that these wells can exhibit sand production from the formation when drilled in non or poorly consolidated reservoirs (turbidites) if the clean up process is not carefully optimized. The questions are: which pressure drawdown has to be applied and according to which procedure?
In this paper, a simplified methodology is proposed to simulate the flow of a single liquid phase, namely oil, in the close vicinity of an horizontal well during its clean up period when a pressure drawdown is imposed between the wellbore and the reservoir formation. This work constitutes a first approach in order to provide qualitative information on the capacity of self cleaning of a horizontal well drilled in an oil pool with an oil base mud. Numerical results are presented that shows the influence of the pressure drawdown imposed during the clean up period on both the velocity of the produced oil at the wellbore and the respective contribution of contrasted permeability zones on the total flowrate of the well. The presence of an external filter cake together with an internal damage zone due to mud filtration is also studied.
Ultimately, these results will be used within a more complete wellbore modelling that will include other features such as sand grain pulling from the reservoir formation.