Determination and Application of MWD/LWD and Core Based Permeability Profiles in Oseberg Horizontal Wells
Authors: Kristiansen J.I. , Sognesand S. , Bergum R. 
Published: December 2007

Quicker and better quantitative partial perforation strategy has been possible by applying more accurate predicted permeability profiles along horizontal wells in the simulation models. A newly developed methodology for predicting permeability in non-cored horizontal wells that uses measurement/logging-while-drilling (MWD/LWD) data and a multivariate technique makes this possible. Data from the Oseberg Field in the North Sea illustrate the technique. The permeability modelling is done with a Partial-Least-Squares (PLS) regression technique. The underlying calibration model is founded on logs and core data from selected wells. Gamma ray, neutron, density, resistivity logs and air permeabilities as measured on core plugs constitute the applied calibration data set in the present context. In subsequent prediction phases, new logging data are fed into the established model to produce the permeability profile.

The underlying model building is briefly outlined. In the actual Oseberg case, various permeability models are established. Their use depends mainly on formation, height above free water level and the data quality. The analysis shows that quantitative permeability profiles can be predicted within an acceptable precision if the input data quality is satisfactory. The predictions are fast and so far, tenth's of wells have been modelled. The enhanced quantitative application of all available log information is the major difference from previous applied permeability prediction procedures. The contrast is use of single log information and qualitative argumentation. The methodology is considered to have a significant contribution towards a tool for quicker and better quantitative partial perforation strategy in horizontal wells completed with cemented liners. Improved areal sweep and thereby increased recoverable reserves lead to cost effective production. It also enhances the possibility to obtain a better pre-knowledge of expected inflow profiles and it adds information that is useful in evaluating subsequent production logged inflow.

Two examples of permeability profiles from Oseberg horizontal wells are given and their applicability for well simulation modelling to predict inflow is illustrated. The results are discussed and compared with production logged inflow. Further applications of the prediction procedure are presently evaluated. This relates to improved field wide permeability mapping and the real-time geosteering aspects.

Introduction

The Oseberg field is one of the giant oil fields in the North Sea (Fig. 1). It is produced from two platforms and production originates in several sand units within the Middle Jurassic Brent Group. The Oseberg Formation that is the main reservoir unit shows an excellent reservoir quality. The overlying Rannoch and Etive Formations have generally poorer reservoir qualities especially the first mentioned. Above these the Ness Formation constitutes an important reservoir, but due to its complicated channel sands it is a more difficult reservoir system. The Tarbert Formation which terminates the Brent Group shows a varying reservoir quality, but to the north it is an important reservoir unit of good quality.

The main production mechanism is gas drive and horizontal wells contribute significantly to the production. Large volumes of gas are injected in the gas cap to maintain the reservoir pressure. This combined with oil production from horizontal wells situated a few meters above the oil-water contact (OWC) lay the foundation of an effective drainage of the field. In addition, a thorough perforation strategy applied to individual production wells ensures an optimal areal sweep.

Well simulation modelling is a cornerstone in the perforation strategy. P. 371