Abstract
Cores can be considered the ground truth only if we eliminate or minimize their damage during the core cutting, tripping, and surface handling. During coring operation, while tripping out of hole, core is submitted to a relatively sudden decrease in pressure, leading to fluid expansion and movement out of the pore space. The rapidly expanding fluids can generate fractures and damage the core. Such damage would adversely affect their properties. In this paper an analytical approach model is introduced and applied to provide the maximal coring retrieval tripping rates. Ideally, core tripping rates should be established for each core, but common practice is based on generic rules of the industry. Traditional operating schemes do not justify the long tripping times.
With these high daily rig costs in mind, a more scientific and quantitative approach, tailored to each case is required. The Thermo-Poro- Elastic (T-P-E) geomechanical approach used in this study includes the diffusion time required for imposed pore pressure difference to dissipate while also considering the effects due to the temperature changes, the mud cake and swabbing during retrieval.
The hydraulic diffusivity and the fluid are the main factors that contribute to the maximum allowable safe tripping rates. A relationship between the hydraulic diffusivity and the decompression rate will be presented for each specified fluid type. For water-bearing cores there is almost no change in hydraulic diffusivity, on the other hand, gas-bearing cores and oil-bearing cores vary with depth.
This paper is a review of T-P-E analytical development and mathematical models to approximate the case of core tripping considering the effects of the pore pressure, temperature change, the mud cake, and swabbing. The model indicates that the fluid type and hydraulic diffusivity have been considered as the controlling factors. The analytical approach has been applied for modeling the tripping of water, gas, and oil-bearing cores to provide maximum allowable tripping rates.
