A semi-analytical approach to model drilling fluid leakage into fractured formation

Loss of circulation while drilling is a challenging problem that may interrupt operations and contaminate the subsurface formation. Analytical modeling of fluid flow in fractures is a tool that can be quickly deployed to assess drilling mud leakage into fractures. A new semi-analytical solution is developed to model the flow of non-Newtonian drilling fluid in fractured formation. The model is applicable for various fluid types exhibiting yield-power law (Herschel-Bulkley). We use finite-element simulations to verify our solutions. We also generate type curves and compare them to others in the literature. We then demonstrate the applicability of the proposed model for two field cases encountering lost circulations. To address the subsurface uncertainty, we combine the semi-analytical solutions with Monte Carlo and generate probabilistic predictions. The solution method can estimate the range of fracture conductivity, parametrized by the fracture hydraulic aperture, and time-dependent fluid loss rate that can predict the cumulative volume of lost fluid.

Post Drill Pore Pressure Prediction for Geo-hazard Assessment of Offset Wells in Hamoru Field

Understanding the distribution and variation of subsurface formation pressure is key to preventing geo-hazards associated with drilling activities such as kicks and blow out. To assess and prevent such risk in drilling offset wells in the Hamoru field, prediction of pore pressure was done to understand the pressure regime of the field using well logs in the absence of seismic data. Two commonly used methods for formation pressure prediction; Bower’s and Eaton’s methods were adopted to predict pore pressure and determine the better of the two methods that will be more suitable for the field. The cross-plot of Vp against density disclosed that compaction disequilibrium is the prevalent overpressure mechanism. The prediction of Pore pressure with Eaton’s method gave results comparable to the acquired pressure in the field, typical of what is expected when compaction disequilibrium is the dominant overpressure mechanism. Since the result of Bower’s method over estimated formation pressure, Eaton’s method appears to be the better choice for predicting the formation pore pressure in the field. Analysis of the predicted pore pressure reveals the onset of overpressure at depth of 2.44 km. The formation pressure gradient ranges from 10.4 kPa/m to 15.2 kPa/m interpreted as mild to moderately over pressure. Keywords: Geohazard, over-pressure, Eaton’s method, Bower’s method, normal compaction trend

Estimation of subsurface formation temperature in the Upper Yangtze Area, Southwest China

<p>Subsurface formation temperature in the upper Yangtze area, southwest China, is significant for assessment of hydrocarbon generation and preservation, especially that of shale gas. The upper Yangtze area, with well-developed marine carbonate rocks, is one of the important preferred areas of shale gas exploration and development in China. Previous studies have analyzed the accumulation mechanism, development characteristics, hydrocarbon generation potential and occurrence modes of shale gas. However, the analysis of subsurface formation temperature is rare due to a lack of highly accurate temperature data. Here we combined new steady-state temperature logging data, drill-stem test temperature data and measured rock thermal properties, to investigate the geothermal regime and to estimate the formation temperature at specific depths in the range 1000~6000 m in this area.</p><p>Our results show that the present-day geothermal gradient for this area ranges from 10 to 74℃/km, with a mean of 24℃/km; While the heat flow varies from 27 to 118mW/m<sup>2</sup>, with a mean of 64mW/m<sup>2</sup>, indicating a moderate-high geothermal regime. Formation temperature at the depth of 1000 m is estimated to be between 26 °C and 71°C, with a mean of 40°C; the temperature at 2000 m ranges from 36~125°C with an average of 64°C; 45~180°C is for that at the depth of 3000 m, and the mean is 88°C; the temperature at 4000 m varies from 88 to 235°C, with a mean of 112°C; 65~290°C is for that at 5000 m depth, with a mean of 136°C; 75~344°C is for that at the depth of 6000 and the mean is 160°C. Generally, the pattern of the estimated subsurface temperatures in different depths is similar and has an obvious sub-area characterization, showing a trend of gradually increasing of temperature from northeast to southwest area. Most areas in the south and southeast of Sichuan Basin are with moderate temperature area, which maybe is the “sweet spot area” for shale gas exploration.</p>

SILVER-GOLD FORMATION TYPE ORE MINERALIZATION IN TAJIKISTAN

Серебро-золоторудный формационный тип минерализации в Таджикистане известен в пределах Табошар-Канджольского рудного узла (Карамазар) и на Памире. В Карамазаре к этому типу относятся месторождения Школьное, Четсу и Караулхона, а на Памире рудопроявления Сассык, Лянгар, Бугучиджилга, Курустык и др. Серебро-золоторудный формационный тип представлен убогосульфидными кварц-золоторудными жилами с высоким содержанием серебра. Рудные минералы представлены пиритом, тетраэдритом, халькопиритом, пираргиритом, фрейбергитом, миаргиритом, самородным золотом, электрумом и кюстелитом. Формационными особенностями этого типа являются предрудная пропилитизация, синрудная березитизация, многостадийный характер минерализации, простой минеральный состав, крайне неравномерное распределение серебра и золота, а также близповерхностное образование золота и его низкопробность. Продуктивное оруденение в них образовалось при сравнительно низких температурах (300–150ο) и давлениях (500 бар и ниже). Silver-gold ore-formation type mineralization in Tajikistan known within Taboshar-Kanjol – ore unit (Karamazar) and the Pamirs. The most known deposits KaramazarScholnoe, Chetsy and Karaulhona and the Pamirs to this type of ore can be attributed Sassyk, Langar, Buguchidzhilga, Kurustyk. Silver-gold ore-formation type is represented by poorly-high silver sulfide-quartz veins of gold mining. The ore minerals are pyrite, tetrahedrite, chalcopyrite, pyrargyrite, freibergite, miargyrite, native gold, electrum and kyustelitе. Formational peculiarities of this type are pre-ore propylitization, sin-ore beresitization, multi-stage nature of the mineralization, simple mineral composition, extremely uneven distribution of silver and gold, as well as subsurface formation of gold and its sleaze. Productive mineralization formed there in at relatively low temperatures (300–150ο) and pressures (500 bar or less).

A simulation study on CO2 sequestration in saline aquifers: Trapping mechanisms and risk of CO2 leakage

Carbon dioxide (CO2) is one of the main greenhouse gases that its high concentration in the atmosphere has caused the global warming issue. Sequestering CO2 in a suitable geological subsurface formation can be a feasible method to reduce the CO2 concentration in the atmosphere. CO2 sequestration in saline aquifers can store a significant volume of CO2 underground for thousand years. However, injecting CO2 into such formations does not guarantee a safe storage because CO2 could leak back to surface or contaminate the formation water. Hence, a proper evaluation of the sequestration site is required. In this study, a case study regarding CO2 sequestration in saline aquifers was conducted using CMG-GEM compositional simulator to study the effects of aquifer permeability, injection pressure and well trajectory on CO2 trapping mechanisms during sequestration process. A field-scale model with one injector well in which CO2 was injected into the aquifer for ten years and simulated for hundred years was studied. The results showed that, CO2 solubility trapping is the dominant mechanism with less risk of leakage when the aquifer has a good vertical permeability and the injection pressure is not high regardless of the well trajectory.

Low-Frequency Seismic Node Based on Molecular-Electronic Transfer Sensors for Marine and Transition Zone Exploration

A self-contained seismic station that has a modular structure adjustable to different operational conditions—like onshore, offshore to 500-m depth, and at transition zones—has been developed and field tested. The station operation frequency band is 1–300 Hz, which is wider than that of the majority of seismic stations based on using standard (10 Hz) geophones. Such improvement was achieved through the use of molecular-electronic transfer seismic sensors that allow for covering a low-frequency part of the spectrum that is needed for broadband processing and receiving information on subsurface formation. Basically, the system includes a module of sensing elements, a module of digital electronics, and a battery module. Optionally, a self-surfacing module could be used. The field test of the station was performed in August 2016 in the Sea of Azov.

High-Temperature Acoustic Fluid Velocity Sensor Development

There are numerous challenges to developing sensors for high-temperature and high-pressure hydrocarbon reservoir fluid analysis applications in the downhole environment. These challenges include material selection as well as processes for manufacturing, testing, and verification of the sensors. We were tasked with developing sensors for the analysis of subsurface formation fluid samples that required reliable operation up to 38,000 psi (262 MPa) and 450° F (232° C). This paper focuses on the technical challenges and our solution for one such sensor that measures the sound speed of the sampled formation fluid. The core of our acoustic transducer is a piezoelectric crystal, which serves as the acoustic transmitter and receiver. This paper-thin, coin-shaped crystal has even thinner silver electrodes deposited on both of its faces. Numerous wire-to-electrode bonding methods were investigated, which included soldering, sintering, and epoxying, the latter of which proved successful.