As you are all aware, our industry is facing—once again—difficult times.
My reason to mention it here is to provide a reminder to the fact that we are an organization of volunteers. And that means, with fewer colleagues around (and tasks and work load not necessarily being reduced proportionally…), the competition for time available for voluntary activities becomes harder. Therefore, I kindly ask for your understanding that the response time (submission to first decision) to our authors shows an increasing trend over the last months (~100 days on average). Please be assured that the Editorial Review Committee and I are working hard on reversing this trend.
And of course, should you now contemplate getting yourself (more) involved volunteering in one of SPE’s various activities—even better!
As stated before, personally I am convinced that we need to keep an open eye (and mind!) when looking for potentially useful new technologies or methods to increase our efficiency. Therefore, without further verbiage, now on to our articles, meant to serve as “food for thought” supporting you in your professional tasks.
Our completion article in this issue of SPE DC covers the most important component of many sand control designs, the (metal mesh/premium) sand screen. And the qualification of stand-alone screens for the intended environment traditionally often involves a whole series of laboratory tests with the associated demands on time and cost.
In Characterizing, Designing, and Selecting Metal Mesh Screens for Standalone-Screen Applications, the authors present a new approach for the characterization of plain square mesh and plain Dutch weave screens by use of micro-CT (computed tomography) scanning. This allows generating a computer-aided design screen coupon model—including the support and protection layers—and its subsequent use in discrete element method simulations for sand-retention performance. Directly generated (i.e., without previous CT scans of actual screens), digital coupon models (“virtual sand screen”) are also investigated, together with a discussion of the relevance of “wire overlap” observed in the physical screen coupons used.
Admittedly, at first glance, the approach looked, perhaps, a bit “nerdy” to me (as somebody who struggles already with his smart phone settings...), but it is in fact a very good example demonstrating the importance not to focus on the filter layer only, and how the use of computation can help narrow down the number of actual laboratory experiments required for stand-alone screen selection—and to save cost by doing so.
If you are concerned with directional drilling, our next article is for you because it describes a series of drilling experiments in an actual test well to improve the understanding of toolface control with different bit types under various load-change scenarios.
The paper The Effect of Bit Type on Reactive Torque and Consequent Tool-Face-Control Anomalies presents drilling perturbation tests (varying weight on bit, rate of penetration, and drilling through formation boundaries with a strength contrast) in slide mode with a 1.3o adjustable kick-off motor bottomhole assembly (BHA) and nine different 8¾-in. bits [polycrystalline diamond compact
(PDC), hybrid, and roller cone]. The respective tool responses (torque changes, toolface hysteresis, etc.), recorded downhole at high frequency, are shared. During the tests, a toolface control anomaly called fast torque anomaly was identified (associated mainly with PDC bits and increases in downhole torque) and the conditions leading to it are discussed in detail.
These useful, full-scale test results improve our understanding of dynamic BHA responses to surface changes and can help explain why we sometimes struggle getting the bit orientation desired.
Our next drilling article describes investigations about the topic of drillstring dynamics (i.e., torsional, axial, and lateral vibration modes), which are important for the understanding of the rather complex response of drilling systems, actual loads on, and the related fatigue risks of downhole tools.
In the paper Continuous High-Frequency Measurements of the Drilling Process Provide New Insights Into Drilling-System Response and Transitions Between Vibration Modes, the authors present how downhole high-frequency data over relatively long durations (minutes to hours) were acquired by means of a BHA mounted sub. Data of six field examples are analyzed (stick/slip, whirl, whirl type, whirl severity, bit bounce, and axial excitation from rig heave).
With an improved understanding of the actual downhole loads, it becomes possible to reduce wear and fatigue—hence, cost—by refining procedures and string designs accordingly.
The following article fits well with the previous one by covering drillstring dynamics, too, (i.e., stick/slip), and it also offers a detailed look into a commonly used method of friction-factor estimation.
The paper Stick/Slip Detection and Friction-Factor Testing With Surface-Based Torque and Tension Measurements describes how high-frequency data were acquired by use of a torque-and-tension sub (TTS) installed directly below the top drive. The surface data from bit runs in horizontal shale-gas wells are subsequently compared with measurements recorded downhole and found in reasonable agreement (< 30 %) for stick/slip monitoring.
In the second part of the manuscript, the authors perform a detailed analysis of hookload measurements with a load cell in the deadline vs. the TTS output. It shows a systematic discrepancy between the two, underlining the importance to consider sheave friction when estimating friction factors from pick-up or slack-off weight indicator readings. Ignoring it can introduce the risk of working with misleading hookload vs. depth plots.
For all colleagues aiming to widen their “drilling window” by wellbore strengthening techniques, the next paper is highly recommended.
In A Review on Fracture-Initiation and -Propagation Pressures for Lost Circulation and Wellbore Strengthening, a detailed overview about fracture initiation (perfect wellbore, effect of microfractures, and comparison with leakoff-test results), fracture propagation (theory, in-situ stress state, influence of pore pressure, fracture plugged by solids, and fluid leakoff through fracture face), and the effects of formation permeability (sand/shale) and capillary pressure (oil-/synthetic-based mud) is provided.
And because it covers all relevant aspects, is relatively easy to understand, and allows readers to draw their own conclusions, in my view, this article could also be well suited serving as a general introduction into the theme “wellbore strengthening” for people not exposed to it on a daily basis.
Obtaining a gas tight seal is the target of most cement jobs pumped. Therefore, measures are taken to prevent gas migration into the cement during early slurry gelation with hydrostatic-pressure decreasing. One parameter used in the industry to describe this reduction in hydrostatic-pressure and to define the so-called transition time (critical hydration period) is static gel strength (SGS).
In Theory-Based Review of Limitations With Static Gel Strength in Cement/Matrix Characterization, authors challenge if SGS, originally describing shear stress at interfaces, is indeed a suitable concept to predict gas-migration potential. The article starts with the SGS theory, presents practical approaches (flow-potential factor, critical wall-shear stress, and slurry response number) and laboratory transition-time measurements, and continues with a characterization of a cement slurry’s microstructure and the shortcomings of the SGS approach, describing it—and its time dependency.
The authors show that using the SGS concept to estimate a gas migration risk into curing cement slurry has limitations. While alternatives will probably not be developed overnight, a critical discussion of what is actually analyzed now can be certainly helpful.
CO2 injection wells, be it for enhanced-oil-recovery purposes or as part of (permanent) storage, are subjected to large temperature variations, especially when operations are intermittent. These temperature fluctuations lead to thermal stresses potentially causing de-bonding of annular barrier material (i.e., cement) with the risk that well integrity can be lost.
In the article Study of Thermal Variations in Wells During Carbon Dioxide Injection, authors present their heat conduction model and how it was calibrated with laboratory experiments. Subsequently, the model is used to simulate the thermal response of four different annular sealing materials with different physical properties (i.e., cement, sand slurry, Bismuth-tin alloy, and a thermally setting polymer) against three different formation types (sandstone, rock salt, and shale).
The manuscript highlights the relevance of investigating alternative annular barrier materials—other than cement—for wells where high thermal stress levels are to be expected.
That’s it for our second issue in 2016. On behalf of the entire Editorial Review Committee, I thank you for your continued support of SPE Drilling & Completion.
Christoph Zerbst is a senior production technologist at Petroleum Development Oman, Muscat, Oman and the Shell Principal Technical Expert (PTE) for Conceptual Completion Design. Most recently, he worked as lead production technologist for Sakhalin Energy in Yuzhno-Sakhalinsk, Russia, and as senior production technologist at Brunei Shell Petroleum in Seria, Brunei Darussalam. Before joining Shell in 2002, he spent 12 years with Wintershall in various production and completion related capacities. Zerbst has worked in several geographic locations mainly in production operations, completions (design and installation), workovers and well interventions (planning and execution) and field development, permanently based and on rotation, for on- and offshore fields. He volunteers as Technical, Associate, and Executive Editor for SPEDC since 2003 and became SPE Peer-Apart honoree in 2010. Zerbst also serves as member of Task Group API Specification 19AC, Completion Accessories. He has a Dipl.-Ing. (M.E.) degree in Petroleum Engineering from Technical University Clausthal, Germany.