Abstract
Forward modelling was applied to correct formation pressures measured while drilling on a wiper run for the effects of supercharging. Supercharging is increased sand face pressure caused by drilling fluid filtrate leak-off. The study is carried on one of the well-known carbonate reservoir of the North Sea. This reservoirs, in general, exhibit good porosity; however they have poor permeability because of small pores.
Pressure variations near the wellbore are primarily influenced by near-wellbore drilling fluid filtrate invasion and filter-cake formation. In general, the lower the sand-face permeability, the higher the variations. Considerable progress has been made towards understanding how filter cake forms and how it influences the near-wellbore pressure stability. Available analytical and numerical models in generally focus on dealing with "initial spurt loss" only, remaining transition and dynamic periods are assumed to be negligible. However the dynamic period, which incorporates possible erosion, plastering, clogging, and other implications, can be modelled if the sand-face (near-wellbore) is exposed to controllable and quantifiable influences. The greater number of planned quantifiable influences, the better the forward modelling. The coupled filter cake growth and formation pressure model incorporates; the geometry of the well and the drilling assembly, the time sequence of the drilling or wiper operation, and drilling fluid and formation properties.
A total of 52 formation pressures were acquired during wiper run, across several thousands of horizontal section drilled in to the chalk reservoir. Pressure tests were evenly distributed to evaluate possible faults, depletion, and pressure barriers, and, more importantly, to calibrate the flow model for the future drilling campaigns. Tests acquired at same depth interval with different circulation rates were used as primary the calibration point for the forward model calibration. A secondary calibration point was obtained by from two consecutive tests, during which first circulation was kept off, and then turned on. These simulations are also applicable to exploring system behavior and responses when planning and executing the job, assessing the feasibility and suitability of the methodology to check that assumptions are satisfied, and building some expectations about the likely measured pressures and their behavior over time.
