High-Accuracy Oriented Perforating Extends the Sand-Free Production Life of Andrew Field

Abstract

BP started development of Andrew field in 1996 with a program for 13 producer wells and one gas injection well. Oriented perforating was used for the weaker Andrew sandstone intervals in some wells, as a means of reducing the risk of sand production. However, the accuracy of oriented perforating was unknown at the time and sanding has recently occurred on some of the wells. Subsequent work with the latest sanding-prediction software showed that perforations oriented away from the vertical direction could have collapsed and provided a source of sand production. Perforations oriented much closer to the vertical direction would, however, be stable and would extend the sand-free field life considerably. This paper describes the results and the evolution to high-accuracy orienting systems on three new Andrew wells and two workover wells. Introduction The Andrew field is in the UK sector of the North Sea, and is a Paleocene sandstone dome structure at a depth of between 2450 to 2555m true vertical depth subsea (tvdss). It has an average porosity of 20% and typical permeabilities in the range of 150 to 1000mD. Initially there was an ~60-m saturated oil column with an overlying primary gas cap and an underlying large and active aquifer (Fig. 1). The field has been on production since 1996 when four predrilled wells were completed. The base development program called for a further 10 horizontal producers and a vertical gas-injection well. These were drilled radially from the Andrew platform at the center of the structure. The producers were perforated underbalanced, with some perforation intervals oriented to minimize risk of sand production. Two infill wells were drilled in 1998. Andrew completion strategy is discussed by Mason and Gomersall. [1] Essentially using horizontal wells with cemented liners was the strategy chosen to allow easy access and water shutoff later in the field life. Studies showed that the formation was sensitive to kill fluids, so a "no-kill" perforating solution was chosen using a downhole isolation valve with lengths in excess of 1000 m perforated on each well. Andrew came off plateau production in 2000 owing to the platform gas constraint (Fig. 2). In 2001, sand production was detected in two wells, which were then choked back. The sand production appeared to be related to pressure drop since water had already broken through to some wells as early as 1998, with some water cuts as high as 50% before sand was detected. With a new well planned (Andrew A-15), the perforating strategy was reviewed. Original Perforations The original horizontal wells were perforated underbalanced by circulating-in base oil before the completion was run, and an isolation ball valve[1,2] provided well control (Fig. 3). The isolation valve was installed just below the packer in the 9 5/8-in. casing with the completion stinging into a seal bore at the top of the 5 ½-in. liner. Once the completion and tree were installed and tested, guns were run through a hydraulic workover unit and located on depth. The guns were fired with the well underbalanced by about 300 psi. As the guns were removed from the well, a shifting tool, located at the bottom of the guns, closed the isolation valve. An inflow test confirmed that the valve was closed and thus allowed the guns to be quickly and safely removed from the well. The well was put on production by opening the isolation valve, using pressure to cycle the control module to the "open" position. Wells A-02 to A-06 used a mixture of 3 3/8-in., 4 shots per foot (spf) guns: full 60° phased guns in the stronger B sandstone intervals; and ±25° phased guns in the weaker A sandstone intervals (Fig. 4). The phased guns were oriented to shoot the high side of the hole, with perforations oriented 25° either side of the vertical. At the time, it was assumed that gravity-assisted orientation would point the guns in the right direction with no consideration for alignment error.

Pore-Level Analysis of the Relationship Between Porosity, Irreducible Water Saturation, and Permeability of Clastic Rocks

“…20,21 Core samples were acquired from a shallow interval of A1-A5 m and a deep interval B1-B5 m. Laboratory measurements on these samples, including porosity, irreducible water saturation, and permeability, were well conducted and documented. Empirical models give a better estimate of permeability for samples with a PT clay growth pattern than for those with a PU growth pattern.…”

Effects of Surfactant-Emulsified Oil-Based Mud on Borehole Resistivity Measurements

“…The field case considered in this work corresponds to a Paleocene sandstone dome located in the central North Sea (Martin et al 2005). The field is composed of an average of 60 m of gas cap with a gas/oil ratio of 871 scf/bbl and 58 m of oil-saturated column with a gravity of 40°API followed by an active aquifer (BP 2003).…”

Fluid Density and Viscosity Effects on Borehole Resistivity Measurements Acquired in the Presence of Oil-Based Mud and Emulsified Surfactants

“…The field case considered in this work corresponds to a Paleocene sandstone dome located in the Central North Sea (Martin et al, 2005). The field comprises an average of 60 m of a gas cap with a gas/oil ratio of 871 scf/bbl and 58 m of an oil-saturated column with a gravity of 40 o API followed by an active aquifer (BP, 2003).…”