Interpretation Of Borehole Data Research Articles

Uncertainty analysis of numerical inversions of temperature logs from boreholes under injection conditions

Abstract Conventional methods to estimate the static formation temperature (SFT) require borehole temperature data measured during thermal recovery periods. This can be both economically and technically prohibitive under real operational conditions, especially for high-temperature boreholes. This study investigates the use of temperature logs obtained under injection conditions to determine SFT through inverse modelling. An adaptive sampling approach based on machine-learning techniques is applied to explore the model space efficiently by iteratively proposing samples based on the results of previous runs. Synthetic case studies are conducted with rigorous evaluation of factors affecting the quality of SFT estimates for deep hot wells. The results show that using temperature data measured at higher flow rates or after longer injection times could lead to less-reliable results. Furthermore, the estimation error exhibits an almost linear dependency on the standard error of the measured borehole temperatures. In addition, potential flow loss zones in the borehole would lead to increased uncertainties in the SFT estimates. Consequently, any prior knowledge about the amount of flow loss could improve the estimation accuracy considerably. For formations with thermal gradients varying with depth, prior information on the depth of the gradient change is necessary to avoid spurious results. The inversion scheme presented is demonstrated as an efficient tool for quantifying uncertainty in the interpretation of borehole data. Although only temperature data are considered in this work, other types of data such as flow and transport measurements can also be included in this method for geophysical and rock physics studies.

Formation of ground fissures with synsedimentary characteristics: A case study in the Linfen Basin, northern China

The Zezhang ground fissure in the Linfen Basin, China was first observed in 2004 where it damaged the infrastructure in and around the town. Field and laboratory investigations now confirm that the fissure has had a complex origin, ranging from likely recurrent fault offset with almost synchronous local sedimentation (synsedimentary), to accelerated groundwater withdrawal, soil erosion and ultimately to geomorphic expression of surface subsidence, local linear scarps and sinkholes. Trench exposures and interpretation of borehole data indicate that at least four tectonic events took place during the latest Pleistocene and Holocene generally localizing the zone of fissure expression. In contrast to other tectonic ground fissures, the ground fissures in Zezhang are highlighted with major synsedimentary characteristics. The formation mechanism of ground fissures was caused by several factors, initial and later underlying fault displacements, initial fissures with land subsidence caused by groundwater withdrawal and widening into linear sinkholes under soil erosion. The Zezhang ground fissure, its investigation techniques and the technical findings, provide a case study for assessing previously unrecognized synsedimentary fissures elsewhere.