One most common cause of damage of the recovered cores occurs during their fast decompression from the bottomhole to the surface. This occurs as sufficient time is not allowed for the pore fluids in the center of the core to dissipate than in the annulus. As a result, excessive pore pressure difference and stresses are induced in the core body. This may cause tensile failure which shows itself as microfractures in its body. These microfractures adversely alter the core rock and mechanical properties.
Therefore, the sample should be retrieved/tripped slowly enough so that the induced pore pressure and stress difference do not become excessive and the core does not undergo tensile failure. On the other hand, this slow enough retrieval rate should meet the operational rig costs so that it does not cause invisible lost time. Therefore, the optimal tripping rate should be determined for each case. This should be used as the basis of coring method/tool candidate selection. The industry has so far utilized only generic methods for selecting their core tripping schedules and thus their coring methods (used for coring tool candidate selection). Just recently, there has been some research in this regard which failed to consider the thermal, mechanical, and mud cake effects, they did not also evaluate the induced stresses and the optimal rates.
Therefore, in this work, a state-of-the-art T-P-E model is developed and applied to evaluate the optimal tripping rates. This work includes: 1) the modeling, derivation, and evaluation of the hydraulic and thermal effects including the neglected ones in the literature, i.e., the thermal, mud cake, and the mechanical properties, 2) summing-up of all the effects to produce the induced stresses, 3) comparing the induced stresses with the tensile strength, 4) indicating whether tensile failure occurs for a specified tripping rate, and 5) repeating the trial and error process for different tripping rates until the optimal rate can be obtained. Therefore, in this work, a standard procedure is proposed to optimize the retrieval rate by requesting a standard set of inputs and running the model for different tripping rates.
In addition, during the modeling process, the contributing parameters have been identified. Having found the contributing parameters, using rig site data including the cuttings analysis (which is used to estimate several input parameters), it is not possible to find the value of the hydraulic diffusivity with certainty. So far, the magnitude of the tripping rate (finding the optimal value of which is the objective of the work) has been controlled just arbitrarily by the drilling crew. Therefore, these two parameters have been introduced as the main uncertain parameters and thus the sensitivity analyses of their effects have been investigated. Using this analysis, some extremes for tripping rates have been introduced, which can be used as a general engineered guideline for coring tool/method candidate selection.