The application of horizontal drilling technique to improve production performance from elected tight chalk reservoirs in the Danish sector of the North Sea has shown to be technically feasible and economically viable.
Maersk Oil and Gas has drilled and completed three horizontal wells with horizontal drainhole sections ranging from 1000 feet to 3000 feet and with displacement of some 9000 feet from the wellhead platform slot.
The essential early planning aspects leading to evaluation and solution of preferred options are discussed. Modifications to the approach adopted as experience is gained and a generalised overview of problems, solutions and costs is included.
Industry reports, in 1985, of successful cementation of a casing string in a horizontal well revived our interest in what we had reluctantly shelved a decade previously. A successful casing/liner cementation was considered a pre-requisite for a multiple fracture completion.
The drilling of our first horizontal well, MFB-14, commenced in March 1987 and two more, MFB-15 and MFB-13, have been drilled and completed since then. While comparisons are difficult, and perhaps subjective, it is of some interest to note that of a total of 41 wells producing from our Dan Field, some 25% of total production comes from the 3 multi-fractured horizontal well completions.
Described below are the initial technical considerations, the evaluation of and resultant changes to the initial programmes and suggested studies and developments of new techniques and tools which we believe will improve the benefits deriving from application of the horizontal drainhole concept in suitable reservoirs.
Reservoir studies were carried out to quantify the potential benefits of both open drainhole horizontal wells and multiple fractured horizontal wells in tight chalk reservoirs.
The conclusions on the relative merits of horizontal wells in tight chalk reservoirs and the Dan Field in particular can be summarised as follows:
A feasibility study on possible applications of horizontal wells in the Dan field concluded that a non-stimulated horizontal well would yield a productivity equal to a successfully propped fraced deviated well while the well cost would be higher.
The PI improvement for a matrix acidized horizontal well compared with an optimally fractured conventional well is only marginal, however. in multi-phase fluid reservoirs where a gas cap and/or bottom aquifer is present, a matrix acidized horizontal well could be beneficial because breakthrough of the gas-water is delayed as a result of the reduction in drawdown. This was studied in detail for Danish chalk reservoirs, including Dan and it was concluded that in the low permeability Danish chalk reservoir coning would still be inevitable for realistic production rates and that no significant improvement in gas-oil or water-oil ratios could therefore be expected although the overall oilrate and recovery would still benefit from the PI improvement.
