Determination of Maximum Allowable Safe-Core-Retrieval Rates

Abstract

Summary Cores can be considered the ground truth only if we eliminate or minimize their damage during core cutting, tripping, and surface handling. Such damage would adversely alter their properties. An important source of core damage is during tripping, when quick decompression might cause damage because of the induced microfractures. In this paper, a state-of-the-art geomechanical model is introduced and applied for determining the safe-tripping rates. The thermo-poroelastic (T-P-E) geomechanical approach used in this study includes the mathematical derivation of the diffusion time required for the imposed pore-pressure difference to dissipate while also considering the effects caused by the temperature changes, mudcake, and swabbing. The work uses different approaches for fluid modeling in a transient manner during tripping for the water-bearing, gas-bearing, and oil-bearing cores. In this work, the hydraulic diffusivity and the fluid type have been introduced as the main factors controlling the maximum allowable safe-tripping rates. A relationship between the allowable decompression rate and the hydraulic diffusivity will be presented for each specified fluid type. In addition, the results indicate that water-bearing cores can be safely tripped as quickly as the normal tripping speed of the wireline, even with a core that has a permeability as low as 0.01 md. For gas- and oil-bearing cores, the safe-tripping rates are determined to be much less than the water-bearing cores because the fluids expand with the pressure drop along the journey to the surface. The results show that the tripping rate is the lowest for the oil-bearing cores, particularly in the vicinity of the bubblepoint and gas critical pressure (because the gas expansion pushes the oil and applies significant viscous forces across the core pore throats). This paper is a novel work developing T-P-E and mathematical models for the case of core tripping considering the effects of pore-pressure change, temperature change, mudcake, and swabbing. The hydraulic diffusivity and the fluid type have been considered as the controlling factors. The approach has been applied for modeling the tripping of water-, gas-, and oil-bearing cores to provide maximum allowable tripping rates.