Safety Of Drilling Operations Research Articles

Uncertainty Evaluation of Wellbore-Stability-Model Predictions

This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 166788, ’Uncertainty Evaluation of Wellbore-Stability-Model Predictions,’ John Emeka Udegbunam, Bernt Sigve Aadnoy, SPE, and Kjell Kare Fjelde, SPE, University of Stavanger, prepared for the 2013 SPE/IADC Middle East Drilling Technology Conference and Exhibition, Dubai, 7-9 October. The paper has not been peer reviewed. The purpose of this work is to investigate typical fracture and collapse models with respect to accuracies in the input data. Uncertainties in the input data will be considered to show how they contribute to the cumulative uncertainties in model predictions. In this approach, the input parameters are assigned appropriate probability distributions. The distributions are then applied in the wellbore-stability models. By means of Monte Carlo simulation, the uncertainties are propagated and outputs, which follow a probability distribution, are generated. Introduction Wellbore-stability analysis is necessary for safe drilling operations, especially now that oil and gas operators venture into more-challenging environments. A wide range of parameters is required for accurate study, many of which are subject to uncertainties caused by measurement errors. Error also can be introduced into data through the methods of interpretation used. Epistemic error, arising from imperfect human knowledge of a system, is another source of input uncertainties. Analytical models used for wellbore-stability analysis are also often associated with uncertainties. Mathematical modeling algorithms only try to approximate physical processes and are not true representations of the problems under study. The modelers should be aware of the imprecision and limitations of these physical models. Thus, output uncertainty stems from the variations in input data and uncertainties caused by wellbore-stability-modeling processes. Expected values give no information about uncertainty. Deterministic estimation of the downhole pressure limits provides only single-point values that lack variability information. Instead, probability distributions can be used. With this approach, cumulative uncertainties in the output predictions can be quantified, leading to a more-informed decision. In-Situ Stress Field For a given formation, the starting point in wellbore-stability analysis is the in-situ or pre-existing stress state. Knowledge of the stress state is key to handling borehole problems such as fracturing, lost circulation, collapse, and sand production. The in-situ stress state is normally assumed to coincide with vertical and horizontal directions. In relaxed depositional basins, the values of these horizontal stresses are usually lower than those of the vertical stress. The horizontal-stress magnitudes, however, may exceed those of the vertical stress in strongly tectonic regions. A stress state can be defined as normal-fault, reverse-fault, or strike/slip-fault state of stress. The normal-fault stress state is assumed in this work. If the magnitudes of the three principal stresses and the direction of one of the stresses are known, then the stress state can be specified. The stress concentration is usually very high around the borehole wall. This effect decreases rapidly away from the hole. At a long distance from the wellbore, the principal in-situ stresses are undisturbed and lie along their in-situ directions.

About this title ‐ Hydrocarbon Exploration to Exploitation West of Shetlands

This volume addresses the challenges facing explorers and developers alike in a region that is becoming a major focus of the petroleum industry in the United Kingdom, Faroes and North Norway. Several West of Shetland fields are still in the appraisal phase almost a decade after discovery. Sub-volcanic exploration risks remain high: sub-volcanic structural traps are imaged poorly, and so the geophysical community is responding with the application of latest technology. The more simple reservoirs might not be large enough to prompt informed and speedy development decisions; larger fields might have a combination of complexities, requiring a phased approach to the development. Infrastructure has been slow to arrive and planned developments have been subject to dramatic swings in fiscal regime ranging from special allowances to unexpected tax increases. Environmental challenges are significant when moving into more remote, deeper water. The perception of these challenges by the third parties has become much more acute. To sustain its right to operate, the industry has to demonstrate safe drilling operations and appropriate response capability with government agencies.

The Reliability and Maintainability Analysis of Pneumatic System of Rotary Drilling Machines

In any blasthole drilling the bottom of the blasthole must be kept clean by evacuating drill cuttings or it flushing as soon as they appear to ensure efficient drilling. If it is not done well, a large quantity of energy will be consumed in regrinding with the consequent wear on drill bit and decrease penetration, apart from the risk of jamming. Therefore, research on reliability and probability of safe operation of pneumatic system of drilling machines is of prime importance to ensure safe drilling operations. In this paper, reliability of this system was modeled and analyzed. To doing this research, drilling machines in Sarcheshmeh Copper Mine in Iran have been selected for data collection and analysis. After reliability modeling of pneumatic system, maintenance scheduling has been presented based on different reliability levels. There were four rotary drilling machines in this mine (named A, B, C and D). Results showed that after about 7 h drilling of machines A and B, and after 103 and 44 h drilling of machines C and D respectively, noticeably the reliability of pneumatic system reached to 80 %. As a result, machines C and D have more reliable pneumatic systems in comparison to machines A and B and checking and servicing of pneumatic system before these time was essential. Also, maintainability analysis showed that more failures of pneumatic system of machines A, B, C and D will be noticeably repaired at about 28, 34, 6 and 9 h.

Quantification of uncertainty in a multistage/multiparameter modeling workflow: Pore pressure from geophysical well logs

Calculating subsurface pressures and predicting overpressured zones, in particular for safe drilling operations, is an integrated approach based on data and assumptions from various sources. Uncertainties arise from the input data and assumptions, but also from the pore-pressure modeling workflow including shale discrimination and the definition of a normal compaction trend line. These stages are usually performed manually by an expert and are prone to subjective, human interpretation. The quantification of pore-pressure uncertainty associated with the manual modeling stages is, therefore, challenging. Algorithms were developed to account for and quantify the resulting uncertainty and enable automated user support of at least parts of the workflow, thus introducing more objectivity into the modeling steps. The first algorithm performs a statistical analysis on gamma ray logs to discriminate between shale and nonshale formations. The second algorithm calculates a series of normal compaction trend lines from porosity-indicating logs from which average pore-pressure models and uncertainty envelopes can be determined. Furthermore, the behavior of trend-line envelopes from the series of trend lines was quantified by a parameter [Formula: see text], which turned out to become constant in the overpressure region. The algorithms were applied to 23 data sets from different regions worldwide. Pore-pressure uncertainty was identified to be in the range of up to 8% for shale discrimination and less than 20% for normal compaction trend-line setting. In addition, pore-pressure uncertainty in the overpressure zone correlated with the [Formula: see text]-factor, which can be used to estimate pore-pressure uncertainty at greater depth from while-drilling measurements in the normal compaction zone. The results also exemplify regional uncertainty variations, which imply that modeling parameters need to be adjusted for specific regions. Moreover, the examples demonstrate that automated algorithms are beneficial methods to add objectivity and reproducibility to the modeling procedure.

Challenging Controversies: A Prospective Analysis of the Influence of New Technologies on the Safety of Offshore Drilling Operations

Integrated operations (IO) is an operating mode in the offshore oil and gas industry that is expected to lead to safer, faster and better operations. This article presents an analysis of the anticipated impacts of increased instrumentation on the safety of drilling operations. The instrumentation is related to the change process of IO, and is exemplified by a group of IO tools for interpretation, diagnosis and automation. An important finding in the study is the identification of a set of controversies that reflect characteristic challenges of drilling operations. These controversies involve the quantity and accessibility of information, the issue of centralized and decentralized control, the relation between standardized and unique interpretation of data, and the heterogeneous nature of engineering work. It is argued that the impact of the IO tools on safety will depend on how these controversies are taken into account when the tools are adopted. It is also argued that the cognitive control of the operations is distributed across a range of human and non-human actors and that the impact of the IO tools thus depends on how they are adapted to the system of distributed cognition rather than on the properties of the tools themselves.

Improving depth prediction accuracy of quantified drilling hazards

Under-compacted shales are often associated with over-pressured formations. These shales have excess water and tend to be mechanically weak (or are overlain by mechanically weak formations), thus the safe mud window for drilling the under-compacted interval can be quite narrow. Efficient and safe drilling operations require accurate depth predictions of these over-pressured formations as well a knowledge of the magnitude of the over-pressure. In this paper we describe a technique which combines the best aspects of conventional Vertical Seismic Profiles (VSP) and Reverse Vertical Seismic Profiles (RVSP) to detect under-compacted shales and predict formation pressures to locate drilling hazards below TD. The excess water in the under-compacted shales will have a lower acoustic impedance than expected from the compaction trend. Shales that depart from the compaction trend may indicate potential drilling hazards below. Conventional VSPs provide at discrete intervals in the well, high quality reflection data which can be used to accurately predict acoustic impedance below the bit. This acoustic impedance is then interpreted to provide both the location (in time and depth) of the drilling hazard and the mud weight necessary to contain it. The two way time estimate of the hazard location is usually quite accurate but the depth estimate is less certain due to the estimation off ormation velocities below TD. The RVSP using the drill bit as a source, provides a continuous time versus depth relationship while drilling. This time verses depth is used to continually update the conventional VSP depth prediction of the drilling hazard and thus provide the most accurate depth of the hazard prior to it's penetration.