Abstract
The Pole-du-Nord Project, offshore Netherlands, covers the joint development of four small Lower Slochteren dry gas bearing sandstone reservoirs. The development scheme is the result of extensive studies to mature an optimized project, taking into account the limited size of the fields, the poor petrophysical characteristics and the reservoirs uncertainties. These fields are located within a circle of 3500 meter (11 500 ft) of radius. The target zone is 60 meters (200 ft) thick and 3700 meter (12000 ft) deep.
Extensive subsurface studies demonstrated that the poor productivity of the exploration wells was explained by facies variations due to sedimentological deposit history. The sedimentary pattern is a mixed fluvial, aeolian deposit that leads to a very heterogeneous multilayered reservoir. The horizontal permeabilities range from 0.01 to 40 mD but the main reserves are located in the poor but thickest fluvial facies, with permeabilities from 0.1 to a 0.5 mD. The drainage of each gas pool by one well was proven with specific stochastic geological modeling constrained by DST results and full field flow simulation.
The development with one central platform was by far the most economical solution. The key factor was the well productivity enhancement.
Various possibilities to enhance productivity were screened, including a horizontal multi-fractured well. The selected solution which honored all the constraints (technical feasibility, minimum cost, initial rate and reserves whatever the reservoir heterogeneity) was a massive fracture treatment on two wells out of the four. The target fracture length was 50 meters (820 ft). The uncertainty in the stress distribution across the reservoir made frac height growth prediction tricky and led to several provisional treatment scenarios.
The target drawdown on reservoir was 200 bar, which made the resin coated proppant selection a crucial point.
The S shape profile was also a drilling challenge regarding the departure of these wells.
This paper presents the project reasoning process, the treatment preparation (including a specific care on the fluid formulation and its interaction with the resin coated proppant) and the fracture evaluation.
The project is a technical and economical success. The lessons acquired in Pole-du-Nord will be a reference for further marginal gas field development in this area.
