Executive Summary

As in years past, please allow me in this summary to express my deep appreciation and sincere thanks to all our editors, without whom there would be no SPE Drilling & Completion.   

The Executive Editor sorts the submitted manuscripts, replies to the authors, and acts as focal point for the journal. But most of the work with our peer reviews is performed by the Associate Editors (AEs). Because they unfortunately often remain “invisible,” it is therefore my pleasure to explicitly and individually recognize these industry experts currently serving as AE, volunteering their limited spare time to ensure quality content is published in SPE Drilling & Completion (only main areas of expertise listed):

  • Bernt AadnøyDrilling design and analysis, wellbore design/construction, research
  • Ramadan Ahmed — Well control, MPD and underbalanced drilling, hole cleaning, research
  • Mark Brinsden — Perforating, completion planning, design and installation, interventions
  • Curtis Cheatham — Directional drilling, drillstring dynamics, drilling equipment
  • Shilin Chen — Bit design and optimization, drillstring dynamics, bit selection
  • Alexander Crabtree — MPD and underbalanced drilling, downhole tools and equipment
  • Barkim Demirdal — Drilling fluid rheology, heavy oil, shale gas, drilling design
  • Lee Dillenbeck — Cement, wellbore integrity, drilling equipment, downhole operations
  • Vassilios Kelessidis — Cutting transport, drilling hydraulics, multiphase systems, research
  • John Mason — Completion planning, design and installation, intervention operations
  • Stéphane Menand — Drillstring design and dynamics, bit selection and performance
  • Kaibin Qiu — Geomechanics, wellbore stability, compaction and subsidence
  • David Stiles — Cement, wellbore integrity, lost circulation, downhole operations
  • Carl Thaemlitz — Fluid chemistry, drilling fluids, handling, processing and treatment
  • John Thorogood — Real-time operations, drilling automation, directional drilling
  • Claas van der Zwaag — Completion planning, design and installation, sand control, impairment
  • “Joe” Yunxu Zhou — Drilling design and analysis, telemetry, automation, drilling hydraulics
Together with our more than 150 Technical Editors (listed individually under Thank You to Our Reviewers) and, of course (and not least), the staff in the SPE office, they take care of this journal with exemplary commitment, knowledge, and energy.

Many thanks to you all!

Now on to our articles, carefully selected and edited with the aspiration to provide “food for thought” in the various areas of your responsibility.


Completion

In wells with openhole sand control, fluids will need to be displaced (almost certainly) at some stage, and engineers have developed—or inherited—various, different approaches to planning and executing this activity. Therefore, I recommend our first paper in this issue because it shares respective “Dos & Don’ts” compiled by one service company after a review of more than 500 completions worldwide (period: 2013–2016).

Best Practices for Wellbore Cleanup and Displacements in Openhole Sand-Control Completions lists the factors governing which fluids are in the well initially and how displacements are performed, and covers chemical compatibility (fluid/fluid, fluid/rock, fluid/hardware) and hydraulic and mechanical (dedicated cleanout string) aspects and displacement stages and techniques. Finally, it discusses several poor practices stemming from assumptions (e.g., about “piston-like” interfaces, screen plugging test results, “solids-free” fluids or pumping rates), which are not necessarily correct.

The authors conclude with clear recommendations and advocate the acquisition of shale samples (often unavailable, because only the pay zone is cored—if at all—in unconsolidated formations) for meaningful fluid/rock compatibility tests. In my view, they do not only provide a welcome guideline for colleagues new to this trade, but also offer seasoned practitioners an opportunity to check if some of their displacement assumptions are indeed justified.


Drilling

One of the important features of drilling mud is forming a decent filter cake with low permeability to minimize formation damage by fluid invasion. Our next paper presents laboratory investigations evaluating how the addition of ferric oxide and silica nanoparticles (NP, mean diameter ≈50 nm/≈12 nm, respectively) to a 7 wt% Ca-bentonite base fluid could help in that respect, additionally extending the bentonite’s temperature stability up to 350°F (177°C).

In Using Ferric Oxide and Silica Nanoparticles To Develop Modified Calcium Bentonite Drilling Fluids, previous research in this area is reviewed, differences between Ca and (the more commonly used) Na bentonite are discussed; experimental procedures are described; and the results of shear response, filtration [up to 500 psi (34.5 bar) differential pressure], and zeta-potential measurements at different NP concentrations are shared. Filter-cake structure, composition, and permeability are examined in detail.

The experiments show that ferric oxide NPs are preferred vs. silica NPs, and that a ferric oxide NP concentration of 0.5 wt% would be recommended for a Ca-bentonite fluid system because, for example, 2.5 wt% can lead to deteriorating cake properties. Additional work will investigate the behavior of ferric oxide NPs in Ca-bentonite WBM containing all mud additives typically used. Perhaps something you may want to consider, especially when facing (deep) formation impairment from WBM filtrate?

Reading of our next paper is highly recommended for every well designer because it presents some very interesting, and, in my view, relevant thoughts about how to assess triaxial stress states of tubulars and connections. Not only does the author share his insights on “traditional” calculation formulae, but also suggests alternative views on how to approach the routine (eternal) question: “Will the selected pipe and connection ‘survive’ under the conditions to which it is exposed?”

A Unified Approach to Yield, Buckling, and Leak in Well Tubulars starts with a review of existing calculation approaches, states the hydrostatic pressure independence of yield and buckling behaviour (= shear phenomena), and describes the three stress invariants involved in yield, buckling, and connection leak. Von Mises stress, fictitious buckling force, and a triaxial leak criterion for threaded connections are discussed, two new leak constants ( α and β) introduced, and example calculations for a 7-in., 35 lbm/ft N-80 API LTC casing connection performed for internal and external leak scenarios (with additional details in the Appendix).

The author concludes that a connection leak would depend on hydrostatic pressure and proposes a new leak criterion, using α and β and a connection safety factor (SF) analogue to the triaxial pipe body SF. Finally, he lists several open questions still to be addressed to validate this connection leak concept. If agreed and adopted, the presented approach would not only allow more cost-effective well designs, but also savings in the area of connection performance evaluation.

Casing wear while drilling is a risk and important in directional wells, not only from a cost perspective, but also from a safety perspective. With our fourth paper, we cover tribological and electrochemical laboratory investigations to improve the understanding of different wear mechanisms caused by tool joints in casings made from various steel grades (in aqueous solutions).

In Study of Oil Country Tubular Goods Casing and Liner Wear Mechanism on Corrosion-Resistant Alloys, the authors review previous research in this field, indicating that wear rates were found higher in corrosion resistant alloys (CRA) than in carbon or low-alloy steel. But why? Grades L-80, P-110, L-80-13Cr, (Super) S13Cr-110, and Ni-based alloy (110 ksi) are investigated by polarization curve measurements and with a ball-on-disk tribometer (100Cr6 and Al2O3 balls) in aqueous solutions of different salt (NaCl) content and pH-value. Photographs of the wear pattern are shown and the results are discussed in detail.

The authors summarize that friction under corrosive conditions results in mild abrasive or corrosive wear, but in severe adhesive wear (scuffing) under noncorrosive conditions (= in CRA casing). Protection might be offered by (nonreactive) ceramic or (corroding) carbon-steel surfaces on the drill string’s tool joints, but both means also have potential disadvantages. Personally, I enjoyed reading this thorough investigation, and learned why special precautions are certainly justified to safeguard any (hefty) investment in a CRA (production) casing.

To be reliably able to close the BOP whenever required is of paramount importance, and our next article shares how condition and performance of this safety-critical equipment (example: pipe ram) could be constantly monitored. Using data typically available from sensors on the BOP stack, an approach with adaptive physics-based models to estimate potential health degradation is presented.

In Real-Time Condition and Performance Monitoring of a Subsea BOP Pipe Ram, after a general description of the BOP system, the selected condition and performance monitoring (CPM) is detailed, starting with a calibrated, high-fidelity model based on the actual configuration of the BOP ram’s hydraulic system, which is used to generate “state coefficients” for the simplified, real-time, adaptive CPM model. Changes in these coefficients allow detection, isolation, and estimation of system degradation and the calculation of a so-called “compliance margin” helpful for maintenance scheduling.

The authors recommend a minimum of 12 closing events to calibrate the CPM model (base line), and show that subsequent variations of the resulting coefficients can be used to identify system degradation (e.g., from increased internal ram friction or hydraulic resistance, sensor bias, or leakage). Because sensor input data are already available, why not make use of such computations (at comparatively low cost) to help avoid potential “nasty” surprises and, in addition, to offer support for BOP pull/no-pull decisions?

Our sixth paper in this issue is related to the first one, but now we are looking in much more detail into the shape and stability of the interface cement/mud and the various factors governing displacement efficiency for complete mud removal to improve cement sheath quality. Based on the concepts of Rayleigh-Taylor (RT) and Kelvin-Helmholtz (KH) instabilities, several displacement cases are studied using computational-fluid-dynamics (CFD) simulations and compared with flow-loop experimental results.

The authors of On the Instability of the Cement/Fluid Interface and Fluid Mixing start with a review of existing literature, describe axial (RT) and radial (KH) interface perturbations, simulate the influence parameters like density, viscosity, surface tension, and flow rate have on interface stability and compare the CFD results with data from flow-loop experiments (i.e., amount of mixing). An example optimization case for an initially inefficient displacement by changing surface tension or density of the displacing fluid is also included.

The ideas of RT and KH were used to build two instability models, which are helpful in predicting interface stability and to quantify fluid mixing. For upward displacement in a concentric annulus, the displacing fluid (= cement) needs higher density and viscosity than the one to be removed. It was also found that an initially inefficient displacement will not necessarily improve by increasing the pump rate, which is not what I (at least) would have expected.

With the next paper, we stay in the cementing arena, because execution of high-quality jobs is—and remains—a top priority for many of us. But chances for a decent cement sheath are “pretty slim” if the slurry does not even reach the desired interval because of losses into the formation. To improve understanding of lost circulation before or during primary cementing, a detailed analysis of 40 offshore well sections is performed, supplemented by laboratory tests with sandstone blocks.

In Understanding Lost Circulation While Cementing: Field Study and Laboratory Research, the authors start with the objectives (When do losses start during cementing?; Do cement slurries have wellbore-strengthening capability?), present the findings of their field study (95% of loss events started before cementing, most while running casing/liner or circulation before cementing; cement-reduced severe loss rates in more than 50% of the cases), and discuss the results of six rock fracturing experiments (different pre-treatment of the blocks) with SBM and cement slurry.

It is concluded that, although typically ECDcmt > ECDmud, the vast majority of the analyzed lost-circulation events actually started before cementing, and that if a section was drilled, cased, and circulated with full returns, the probability to induce losses later while cementing is rather low. In addition, cement slurry was found able to increase the fracture strength of the tested rock. And I am grateful that the authors share these results, allowing adjustment of some not so uncommon “beliefs.”

That’s it for this first issue in 2018. On behalf of the entire Editorial Review Committee, I thank you for your continued support of SPE Drilling & Completion.

Christoph Zerbst, Executive Editor SPE Drill & Compl,
Shell ( christoph.c.zerbst@shell.com)