It is my honor to introduce Shilin Chen (Halliburton) as the next executive editor (EE) of SPE Drilling & Completion (SPEDC). Having served together with him on the Editorial Board since 2012, I know that he will take excellent care of our journal, and as such, I do look forward to the forthcoming SPEDC issues.
I am sharing some journal performance statistics below, covering the period July 2017 to June 2018 (period 2016/17 in brackets).
My term as EE of SPEDC ended at the SPE ATCE 2018 in Dallas (a major industry event certainly well-worth attending), and I am grateful for this highly rewarding experience. It has been my privilege and pleasure to be able to represent the many volunteers peer reviewing for SPEDC and to work with some of the best people in our industry, be it SPE Advisory Committee members, authors, or technical, associate, and executive editors. Additionally, it has been a pleasure to work with the staff (past and present) in the SPE offices in their pivotal roles for this journal: Coty Cater, Jane Eden, Leah Guindon, Stacie Hughes, Judith Martis, Rebekah Stacha, Wendy Tilley, and Jennifer Wegman. A big “Thank you!” to all, and please continue your support.
Now on to our articles, carefully selected and edited with the intent to offer helpful ideas for some tasks in your area of responsibility.
Developing shale gas reservoirs in tectonically complex settings with natural-fracture systems present can be challenging and can lead to potentially disappointing results in regard to proppant placement or well productivity. Therefore, our first article shares an approach to improved geomechanical understanding from onshore China.
Construction of a 3D Geomechanical Model for Development of a Shale Gas Reservoir in the Sichuan Basin describes the work flow for model construction (capturing anisotropy of mechanical properties and in-situ stresses, pore-pressure prediction, incorporation of natural-fracture systems) and the manner in which it was applied for treatment pressure analysis, identification of critically stressed natural fractures, and prediction of potential zones of mud losses during drilling. The authors conclude that honoring anisotropy and proper incorporation of natural fractures of different scales and mechanical properties is essential. Because these overtake the minimum horizontal stress as a controlling factor in zones with natural fractures (i.e., for proppant placement, well trajectories, and perforations), hydraulic-fracture stimulations need to be planned accordingly. In my view, this is another good example of how computation of previously acquired (= available) data can improve our prediction accuracy.
With our next paper, we stay in the arena of unconventional shale resources by investigating the potential benefits of extreme-limited-entry (XLE) perforating for a more-even proppant distribution to improve productivity. Examples from tests in a horizontal well equipped with distributed temperature/acoustic sensing (DTS/DAS) in the Middle Montney formation onshore Canada are discussed.
In Extreme Limited-Entry Design Improves Distribution Efficiency in Plug-and-Perforate Completions: Insights From Fiber-Optic Diagnostics, the authors start with the problem statement: toe-side perforation clusters in a stage remained understimulated despite limited-entry (LE) perforating. They then describe the XLE design for the trial lateral (7,215 ft MD): 2–3 holes/cluster, 3 clusters/stage, 0.39-in. entrance hole diameter, and discuss the pumping pressure and DAS response during the job. The authors emphasize the importance of keeping treatment pressure at maximum level during the job (continually increase rate), which can improve proppant distribution per cluster (+40%) as well as “acoustic cluster activity” (+30% at IP90), used here as a proxy for inflow distribution. Because there is not much downside in designing for higher pressure drop in the perforations—within the strength envelope of a well—perhaps XLE is something for you to consider trying in your asset.
Concerned with hydraulic-fracture stimulations, you might have wondered if actually-contributing fracture (half-) lengths are indeed as long as predicted. Models often assume average proppant velocity from flow equals average carrier fluid velocity and settling velocity from Stokes’ Law; our next paper offers a more-refined simulation approach for proppant transport in a single fracture using computational fluid dynamics (CFD) and discrete element method (DEM).
After a literature review about solid-sphere transport and settling in fluids, A Study of Proppant Transport With Fluid Flow in a Hydraulic Fracture describes the selected simulations (CFD for fluid force, DEM for individual particle motion, CFD/DEM coupling, boundary conditions, verification) and calculates dimensionless proppant velocities in the direction of fluid flow for different input parameters, all found with maxima at dproppant / wfracture = 0.8. The CFD/DEM correlations are incorporated into a hydraulic-fracture simulator and two cases are discussed. It is shown that for the dominant, main fracture created, the CFD/DEM approach—well-suited to reflect particle motion—has hardly any influence on the modeled effective fracture length because of (usually) d / w << 0.8. However, this ratio could be larger for proppant transport in narrower natural-fracture networks. Again, a valuable example of how computation at relatively low cost can help us check the validity of some assumptions.
For all colleagues concerned with sand control, especially openhole gravel packs, I strongly recommend our fourth article in this issue because it describes in detail how downhole gauge data can be used to determine pack efficiency (PE) and to assist in troubleshooting.
The authors of Downhole-Gauge-Data Analysis of Openhole Gravel-Packing Treatments: Method and Examples begin with a review of the available literature, before discussing potential pitfalls in PE calculations, types of gauges, their position, and the merging of data from different sources. An analysis method is proposed: (1) Calculate PE from surface data; (2) use actual completion sketch to identify key components and volumes in between those; (3) analyze openhole displacement stage (after screen installation); (4) analyze circulation tests (prior gravel packing); (5) analyze gravel-pack-treatment data); then, Item 5 is illustrated with four examples, comparing surface with downhole pressure records. The authors conclude that a thorough analysis requires input data from several sources, but “apart from a few very special ideal scenarios, it is not possible to infer pack efficiency from the surface-pressure data alone”. I regard this paper as a highly welcome compilation for systematically analyzing downhole gauge data, thereby supporting gravel-pack optimization.
Our next paper is a “must read” for all colleagues involved in well-trajectory planning and drilling. Well spacings, both on surface and downhole, have become narrower over the years, not only offshore, but also onshore because of well pads and infill drilling in already-developed fields; therefore, the risk of unintended well intersections has increased. The SPE Wellbore Positioning Technical Section (WPTS) has been in existence since 2004, and the consensus of industry operator and service-company experts that are active in this group is shared here.
Well-Collision-Avoidance Management and Principles presents examples of actual well collisions to discuss the underlying causes and actions that would have likely avoided these incidents. Then, minimum-allowable-separation distance and factor are defined before eight collision-avoidance elements are recommended, discussed in detail, and respective work flows are described. The authors summarize that well collisions occur infrequently, but more often than widely assumed. The reasons for these collisions are grouped into the main categories of data, collision-avoidance management, and communication. Personally, I appreciate this “distillation” of years of valuable WPTS work and look forward to the authors’ companion paper SPE-187073-PA (Well-Collision-Avoidance Separation Rule), which will publish in the next issue of SPEDC.
If interested in directional drilling, I recommend our sixth paper to you. When slide drilling with steerable motors, tool-face disorientation can become a challenge, and oscillating rotation pulses from surface have been used to assist in correcting bit orientation. The authors describe their drillstring model and the simulations performed for a better understanding of under which conditions low-frequency string-rotation pulses from surface can improve tool-face control—and when they cannot.
Theoretical Study of Tool-Face Disorientation Mechanisms During Slide Drilling and Correction by Surface-Rotation Pulses briefly reviews existing work in the area before presenting the model developed (drillstring dynamics with mixed friction), its boundary conditions, and the solution algorithm. The authors discuss the mechanism leading to disorientation and the effect of rotation pulses from surface on the basis of computed results. They conclude that tool-face hysteresis (reactive torque accumulates and unloads in different ways) is an essential factor for disorientation, although less pronounced in softer formations. Surface pulses can only be effective if they are able to drive the “vanishing point of reactive torque” down to the bit. In my view, information well worth considering because of its potential for shortening any periods of respective “trial-and-error”.
For all of you interested in water-based-mud (WBM) filter-cake properties, our next paper describes how some are influenced by the addition of Fe2O3 nanoparticles (NP, davg < 50 nm). After investigation of the general feasibility in a simple bentonite suspension in SPE-178949-PA, the authors now test an actual Ca/bentonite WBM system with common additives (viscosifier, filtrate control, thinner, alkalinity agent, CaCO3, and Mn3O4 as bridging and weighting material) included.
Effect of Ferric Oxide Nanoparticles on the Properties of Filter Cake Formed by Calcium Bentonite-Based Drilling Muds describes the experiments performed; discusses filter-cake characteristics dependent on NP concentration (0.3–1.0%wt), differential pressure (200–500 psi for static filtration), temperature (200–350°F), and dynamic filtration (100 rev/min, 500 psi, 250°F); and compares the filtration results with literature data for alternative NP (cellulose, SnO2, Al2O3, SiO2). The authors recommend an Fe2O3 NP concentration of 0.3–0.5%wt, which leads to favorable filter-cake characteristics (smoother morphology, less cake porosity) and reduced filtrate invasion under both static and dynamic filtration conditions. This may be an approach you want to consider trying with bentonite-based WBM for reduced formation impairment from mud filtrate.
I recommend the following paper for all concerned with wellbore stability while drilling, because it describes the analysis of detrimental effects from weak bedding planes in shales. While these considerations are not new as such, the authors provide a welcome “refresher” together with a useful case history from onshore Ecuador.
In paper Wellbore-Stability Analysis Considering the Weak Bedding Planes Effect: A Case Study, weak bedding planes are characterized (Coulomb failure criterion), their effect on wellbore stability is discussed, and the theory for the solution approach is explained (Kirsch, Cauchy), with details about tensor rotation in the appendix. The case study compares wellbore breakout limits with and without consideration of weak bedding planes. The authors conclude that Coulomb’s failure criterion can be used to capture the effect of weak bedding planes on the mechanical strength of rocks. Their presence does not only change the breakout limit, but also the safest drilling direction (least effect if the wellbore axis is perpendicular to the bedding planes). Revising the well trajectory and increasing mud weight allow drilling through a troublesome shale (in strike-slip stress regime) without running an additional liner. Because mainly computation is required—at comparatively low cost—you might consider the influence of depositional beds for improved predictions from wellbore stability analyses.
In our last paper of this SPEDC issue, we present another helpful case study, now about extended reach, horizontal, geosteered infill drilling (ERD) in an Australian offshore oilfield. The authors highlight the evolution of well designs, technology, and operational practices over time, which enabled adding reserves, maintaining production levels and thereby extending field life.
Extended-Reach Drilling To Maximize Recovery From a Mature Asset: A Case Study starts with a description of the reservoir and original and recent well designs; shares many technical and operational details of the drilling and sidetracking campaigns performed in 2008 (2 wells), 2010 (3 wells), 2013 (2 wells) and 2015/2016 (3 wells); and discusses the drilling fluids used throughout (WBM, with xanthan and diutan gum viscosifiers). The authors summarize that it was possible to mitigate rising equivalent circulation density and manage increased wellbore collision risks. Key success factors stated are high-quality data acquisition, “24/7 geosteering” team support, TAML Level 5 system availability for existing wellbores, low friction centralizers, engineered drilling fluids, and (not least!) staff continuity. In my view, a very welcome sharing of what can be achieved in mature offshore fields to access volumes still undrained.
That’s it for our last issue in 2018. On behalf of the entire Editorial Review Committee, I thank you for your continued support of SPE Drilling & Completion.
Kind regards and Auf Wiedersehen,
Christoph Zerbst, Executive Editor SPE Drill & Compl,