Visualisation of Drilling-Induced Formation Damage Mechanisms using Reservoir Conditions Core Flood Testing

Abstract

Reservoir conditions core flood tests have been carried out to examine the damage arising from overbalanced drilling-in operations. Gannet field cores were exposed to a number of water- and oil-based field muds currently in use by Shell Expro. Results showed that the initial permeabilities to crude oil were dramatically reduced after exposure to overbalanced mud and subsequent back production of crude oil. Thin section petrography and SEM/XRD analysis of the cores clearly showed the source of impairment in all cases to be a thin layer of mud solids plugging pores and throats exposed to the wellbore face, forming an effective permeability barrier that was not dislodged by crude drawdown under realistic drawdown. The results were confirmed by trimming of 1 cm from the wellbore face of the core that led to a restoration of initial permeability. Introduction The issue of formation damage has returned to prominence with the exponential growth of horizontal wells in recent years. The increase in productivity offered by the increased contact between reservoir and wellbore is a two-edged sword. The increased contact also allows an increased area of damage to form. The relative effects of that damage on production remain unclear. Near wellbore damage impairs the formation 1s ability to conduct hydrocarbons to the wellbore Formation damage lowers well productivity, defers production revenues and is difficult to repair. A large number of formation damage mechanisms arising from the interaction of drilling mud solids and fluids with the reservoir solids and fluids can occur. Current field evaluation techniques to measure in-situ formation damage may lack the resolution to provide meaningful data on the extent of the problem. Reservoir conditions core flood tests provide a means to simulate the interactions that take place between the invading drilling mud and the formation. This allows measurement of the extent of the impairment and visualisation of the mechanism. The technique attempts to simulate, as closely as possible, the conditions occurring in the reservoir during drilling-in through the use of:–Real, used field muds sampled from previous drilling campaigns–Fresh, well-preserved core material from the target reservoir, and if possible, original native pore fluids, i.e., both pore water and hydrocarbons–Realistic, long-duration core exposure to the drilling fluid under reservoir conditions, with mud flow past the core face at realistic borehole velocities–Measurement of formation damage by crude oil return permeability determination after simulated back production under drawdown P. 101

A New Approach To Estimate Invasion Radius of Water-Based-Drilling-Fluid Filtrate To Evaluate Formation Damage Caused by Overbalanced Drilling

“…Longeron et al (1995) performed experiments to assess the formation damage by drilling and completion fluids. Ghofrani et al (1996) used damage ratio to evaluate formation damage in a laboratory. Hodge et al (1997) evaluated different drilling fluids to minimize formation damage.…”

Modeling of borehole Stoneley waves in the presence of skin effects

“…We choose to specify the impairment layer in terms of the impairment mobility, imp / ͓mD/cp͔ for a reference thickness dϭ0.5 mm, which is the same reference thickness used in the experimental work of Francis et al ͑1995͒ to measure impairment in the laboratory. Note that Eq.…”

“…The layer is not removed by drawdown of crude oil during production and can only be removed through extensive and time consuming cleaning measures. The laboratory core-flood experiments carried out under reservoir conditions by Francis et al ͑1995͒ show that the impairment layer permeability is a factor of 100 to 20 000 less than the formation permeability. The thin impairment layer reduces the average core permeability with a factor of 2 to 150.…”

Improved Lost Circulation Treatment Design and Testing Techniques Minimize Formation Damage

“…It is also worth considering that in the case of formations with high permeability and fractures, both particle and fluid invasion could take place resulting in more severe formation damage. Francis (1995) in his work states that the fine layer of internal residual solids is the prime culprit for the impairment of near wellbore flow potential. One can also find significant work done to assess the magnitude of formation damage because fluids invasion was found to be playing an equally damaging role.…”