Wellbore Stability Simulations for Underbalanced Drilling Operations in Highly Depleted Reservoirs

Abstract

The need to increase productivity reducing drilling formation damage favours the use of underbalanced drilling technology. The main idea is to drill with equivalent circulating densities (ECD) below the formation pore pressure and to avoid the contact between the drilling fluid and the formation. In highly depleted reservoirs, pore pressures can be very low. Therefore, extremely low density fluids such as foams are used to achieve circulating densities below pore pressure. In such cases, the induced modification of the in situ stresses has to be supported mainly by the rock, with low contribution of the drilling fluid pressure. In that sense, the application of underbalanced drilling depends, among other factors, on the mechanical stability of the drilled formation. In general, poorly consolidated depleted formations are not suited for that technology. This paper presents the wellbore stability simulation performed in order to establish the feasibility of using underbalanced drilling in highly depleted reservoirs in western Venezuela. The in situ stress field and the mechanical properties of the formation were obtained. Pore pressure as low as 800 psi at 5500 ft (2.7 lb/gal equivalent fluid density) was measured. The finite difference method and an elastoplastic constitutive model was used to obtain the new stress, deformation and pore pressure distribution. The undrained condition (immediately after the wellbore is drilled) as well as the drained condition were analysed. The analysis showed that horizontal wells could be drilled in an underbalanced condition with low instability risk. Following the recommendations, four horizontal wells were drilled in underbalanced conditions. Values as low as 2.0 lb/gal ECD were used to drill the wells and no wellbore instability problems were reported. Production tests showed an enormous increment in well productivity index in comparison with conventional overbalanced drilling. Introduction Years of crude exploitation can lead to enormous reservoir pressure decline in mature fields, leaving huge amount of oil still in place that can only be exploited with new technologies. Once a mature field has reached an extremely low pore pressure, the target formation should be drilled in an underbalanced condition (using a drilling fluid pressure below the formation pore pressure) to reduce the risk of lost of circulation, but most important, to reduce formation damage and increase productivity. Pore pressures as low as 800 psi at 5500 ft (TVD) has been measured in matured fields. This is equivalent to an hydrostatic column of drilling fluid of 2.7 lb/gal density. To reach the underbalanced drilling condition in such a depleted reservoir, the equivalent circulating density (ECD) of the drilling fluid must be less than 2.7 lb/gal. Conventional drilling fluids are out of this range, therefore the use of low density drilling fluids such as foams is needed. Drilling fluids must, among others, provide good cuttings transport. In addition, when the hole is drilled, the in situ original stresses change near the wellbore and the drilling fluid pressure (or density) must "replace" somehow the support lost by removal the original volume of rock. In conventional overbalanced drilling, the drilling fluid pressure (or density) is usually high enough to provide this support. On the other hand, it is necessary to establish whether low density fluids used for underbalanced drilling in highly depleted reservoirs will provide the pressure needed by the formation to keep it stable. If not, underbalanced drilling is not technically possible and the minimum drilling fluid density (and pressure) will be established by the formation minimum collapse pressure obtained with rock mechanics approaches.