Perforation Stability Analysis as a Tool for Predicting Sand Production: Application for the Brage Field

Abstract

Abstract The Brage Field, located in the northern Norwegian North Sea, currently produces 122 000 BOPD from two separate sandstone reservoirs in the Statfjord and Fensfjord Formations. To investigate the potential for sand production from the Fensfjord reservoir, numerical simulations of stresses and strains on perforation tunnels were performed using 3D Finite Element Modelling (3D-FEM). The material properties of the reservoir rock were determined from laboratory triaxial tests on selected core samples. Boundary conditions were extracted from large scaled model grids and applied to smaller scale models, allowing modelling of a deviated well within justifiable computation times. The numerical simulations predicted local rock failure for the weakest rock in the envisaged drawdown and depletion scenario. Sand production experienced from the Fensfjord Formation supports this conclusion. The observed sand production appears to be linked to high water cut, which is not obvious from the numerical simulations. Field observations are used to verily and calibrate the model. The method is demonstrated by an example analyzing the perforation stability for an inclined well in an anisotropic stress field. Introduction The Brage Field is located in the south-western part of Block 31/4 in the Norwegian North Sea, and was brought on stream in August 1993. An integrated platform produces 122000 BOPD from two separate Upper Jurassic sandstone reservoirs in the Statfjord and Fensfjord Formations. The reservoirs are normally pressured, and the OWCs are at 2149 mTVD in the Fensfjord reservoir and 2381 mTVD in the Statfjord reservoir, both depths referenced to Mean Sea Level. A map of the reservoirs and the platform location is given in Figure 1. Horizontal stresses in the reservoir and overburden have been measured by Extended Leak-Off and Minifrac tests and found to be in agreement with Breckel's and van Eekelen's empirical relationship for the North Sea. Wellbore breakout analyses indicate a small horizontal stress anisotropy with a WNW-ESE trend for the maximum horizontal stress. In the vicinity of faults some stress rotation is observed, in that the maximum horizontal stress is parallel to the fault strike. The WNW-ESE trend is in agreement with the regional trend in the northern North Sea as observed in other fields. The Fensfjord sandstone reservoir, which is the objective of this paper, consists of a shoreface / shallow marine deposit with horizontal permeabilities ranging from 1 mD to in excess of 1 D. The degree of diagenetical cementation is very limited. Thus the mechanical strength of the reservoir rock is governed by mineralogical composition, porosity, grain size distribution and mechanical compaction. The reservoir however contains several horizontal calcite cemented stringers up to 2 m in thickness. These vary from scattered concretions to continuously cemented layers correlatable between neighbouring wells. Some thin >1 m), highly permeable (< 1 D) streaks are present, often directly above or directly below a calcite cemented interval. This may indicate that some high permeability streaks are caused by strain localization during tectonic deformation of the sediments. Expected recovery rates are higher for the Statfjord reservoir than for the less productive Fensfjord reservoir. Thus producing the Fensfjord reservoir as much and as early as possible will give a better total oil recovery from the field. P. 167