Abstract Underbalanced drilling (UBD) is defined as a drilling operation where the pressure of the circulating drilling fluid is lower than the pore pressure of the target formation of interest. The most widely recognised benefit of UBD is the reduction of formation damage by minimising the drilling fluid leakoff and fines migration into the formation. It also facilitates the possibility for reservoir characterisation during drilling. The purpose of this paper is to present (i) how to use the inflow data for the evaluation of formation properties and (ii) how to cope with the uncertainty of the results.
An in-house multiphase reservoir simulator is used for the simulation of the formation reservoir fluid flow during UBD. The model incorporates discrete consideration of the well with appropriate, time varying UBD boundary conditions. Capillary forces, which facilitate countercurrent imbibition of the drilling mud into the formation, are taken into account.
The production rates during UBD depend on the formation properties and the drilling conditions. The inflow information is analogue to transient test data and can be used for the estimation of the reservoir model parameters. With the advancing of the drilling process, the amount of available inflow data increases and allows for a sequential history match procedure. Based on a statistical analysis of the inverse problem we identify the optimal level of parameterisation as quality and quantity of the measured data justifies.
Technical contributions of this paper include (i) the usage of a reservoir simulator to derive the inflow of fluids in heterogeneous reservoirs and (ii) its coupling with gradient-based optimisation techniques for determination of certain reservoir parameters. The "simulation while drilling" approach proposed is fully automated and can set-up on a known or totally unknown reservoir model, introducing an iterative and automatic updating process of the reservoir model.
Introduction In recent years, UBD has become a popular technology. Originally utilised to prevent drilling problems, its value on the reservoir engineering aspects is more and more recognised. Benefit of UBD includes the reduction of formation damage by minimising the drilling fluid leakoff and fines migration into the formation. Additionally, it also allows for reservoir characterisation during drilling.
Since the wellbore pressure is lower than the one in the reservoir, there is a permanent inflow of reservoir fluids during the underbalanced operation UBO (and, possibly, an outflow of drilling mud due to countercurrent imbibition1). The inflow rates can be obtained by balancing injected and produced fluids. In-situ downhole conditions like temperature and flowing wellbore pressure are usually continuously measured while drilling. Applying inverse modelling methods, it is now possible to determine reservoir parameters such as permeabilities and pore pressures by extracting the productivity signature of the reservoir out of the production data. Although the short drilling periods only give insight into the nearer wellbore vicinity, valuable information about the reservoir can be obtained in this way. Productive reservoir zones and formation properties can be determined provided there is a proper flow monitoring at the surface. This offers significant benefits in both the production optimisation and reservoir characterisation but also in the justification of UBD.The objective of our present paper is to demonstrate the usage of a reservoir simulation tool, coupled with a parameter identification method for the purpose of automated reservoir characterisation during drilling. In particular, the tasks from the reservoir engineering perspective, tracked here, are:Detection of the layering,
Identification of layer permeabilities (or other layer properties) and, optionally,
Assessment of the reservoir formation damage