Rotary Sidewall Coring—Size Matters

1 Rotary Sidewall coring size Matters Abhishek Agarwal A formation sample, acquired from downhole, provides a wealth of information Sugar Land, Texas, USA. about rock properties not readily available elsewhere. Rotary Sidewall coring is Robert Laronga one alternative for acquiring downhole rock samples, but in the past, the small Clamart, France sample size often limited laboratory evaluation. A new Rotary coring tool, with Larissa Walker Shell Appalachia Exploration features that improve Sidewall coring operations, addresses core size limitations Sewickley, Pennsylvania, USA by offering larger diameter samples. Oil eld Review Winter 2013/2014: 25, no. 4. Copyright 2014 Schlumberger. For help in preparation of this article, thanks to Chad Albury, Ryan Chapman, Lenishan Fernando and Farouk Hamadeh, Petrophysics is the branch of geology concerned samples, geologists and engineers can perform Perth, Western Australia, Australia; Joe Loman, Dacey with determining physical properties and behavior more experiments and more easily determine geo- McManus and Chris Tevis, Houston; William Murphy, e4sciences, Sandy Hook, Connecticut, USA; Tim Sodergren, of reservoir rocks.

2 Some petrophysicists specialize logic, geomechanical and petrophysical proper- Salt Lake City, Utah, USA; and Benjamin Wygal, Natchez, solely in interpreting remotely acquired measure- ties. Because of the global proliferation of Mississippi, USA. CST, FMI, GPIT, MDT, MSCT, TerraTek, TerraTek HRA, ments in the form of logs; others prefer having a unconventional resource developments, the option TerraTek TRA and XL-Rock are marks of Schlumberger. piece of the reservoir rock along with logs. Those for taking representative samples without the HRSCT and RSCT are marks of Halliburton. with a preference for analyzing rock samples pre- expense and loss of ef ciency associated with con- MaxCOR and PowerCOR are marks of Baker Hughes. fer large ones over small ones. Using larger rock ventional coring has arrived at a crucial time.

3 30 Oil eld Review coring Bit Slabbed Cores Boxed Cores Biscuit Cut 1 ft 1 in. > Conventional coring . Used with a drilling rig, a bit with an open throat (top left) cuts conventional cores. The cores are retrieved at the surface, boxed (left) and shipped to a laboratory for analysis. At the laboratory, technicians typically cut the core lengthwise (middle), referred to as slabbing, to access representative rock for testing. Tests, such as compressive strength measurements along the red lines shown on the slabbed core, may be performed at this stage. After testing the slabbed core, technicians usually cut plugs for additional evaluation. A biscuit cut, which is a slice of core material (right), is an option to taking core plugs from slabbed sections; these sections of core material provide alternative testing options at the core laboratory.

4 (Photographs courtesy of Tim Sodergren.). In the oil and gas industry, operators use two After an interval has been cored, the drilling to acquire core data should conventional coring main methods to acquire rock samples from the crew pulls the pipe back to the surface and operations fail. Because of their small size relative subsurface: cutting whole core with the drilling retrieves the core barrels; the core barrels can to conventional core, there is a chance that SWCs assembly and taking Sidewall samples from the also be extracted from the downhole assembly by taken from a heterogeneous formation may not borehole. Sidewall cores can be further divided mechanical means, which allows the bottomhole have properties that are representative of the for- into percussion and Rotary cores. Drill cuttings, hardware to remain downhole for continued mation at a reservoir scale.

5 The rock from which pieces of rock ground up by the drill bit and cir- coring . Technicians take great care to avoid dam- the SWC is taken may also lack crucial features culated back to the surface, are another source aging the sample inside the carrier after it is that geologists need to analyze the reservoir, espe- of downhole samples, but they may be unreli- retrieved back to the surface. Conventional coring cially in laminated sand-shale sequences, organic able for determining downhole rock properties operations often provide the best rock samples shales and fractured reservoirs. because of the damage they incur during drill- for testing, analyzing and evaluating reservoirs. Two types of Sidewall coring are available . ing and because of depth uncertainty related Compared with normal drilling operations, how- percussion and Rotary .

6 Percussion coring tools, to circulation. ever, the conventional coring process is time rst introduced in the 1930s, use hollow bullets Whole cores, also referred to as conventional consuming and expensive. red into the formation using an explosive cores, are continuous sections of reservoir rock Sidewall cores (SWCs) are plugs of rock taken charge. Rotary SWCs are acquired with a tool cut with a hollow coring bit (above).1 As the bit perpendicular to the wellbore; they are usually that uses a small bit to cut plugs from the side of penetrates strata, a cylindrical section of the acquired by tools attached to wireline. Sidewall the borehole. Rotary coring tools were devel- rock passes through the bit and remains inside a cores can be recovered relatively quickly from oped primarily to address shortcomings of per- core barrel, which is part of the drilling bottom- downhole and may cover multiple zones of interest cussion SWCs.

7 Hole assembly. Whole cores are usually cut in in the same wireline descent; they provide a cost- This article focuses on the development of multiples of 9-m [30-ft] lengths. In deepwater effective alternative to conventional cores. One Sidewall coring and reviews developments in the appraisal wells, it is not uncommon to cut as convenience of SWCs is that they are taken after Rotary Sidewall coring technique. Case studies much as 81 m [270 ft] of core on a single trip in logs have been run, allowing petrophysicists to be 1. For more on whole cores and core analysis techniques: the well. Longer coring intervals come with the selective they can pick core depths based on Andersen MA, Duncan B and McLin R: Core Truth in added risk of jammed core barrels and damaged interpretations from openhole logs. Sidewall cores Formation Evaluation, Oil eld Review 25, no.

8 2. (Summer 2013): 16 25. core sections. also offer an alternative means for petrophysicists Winter 2013/2014 31. A. Combo Bullets Hard Rock Bullet B 7/8 in. 11/16 in. 11/16 in. C Cutting Rings E. 0 in. 2. D. Steel cables > Percussion Sidewall coring . Core bullets are red from a core gun (A) using explosive charges behind each bullet. Various types of bullets are available, including combo bullets (B), which are for medium hard to soft formations. A groove around the top of the bullet accommodates a cutting ring (C) that is held in place with a snap ring (not shown). The type of formation dictates which cutting ring is employed. In soft formations, larger cutting rings reduce bullet penetration. The hardened steel, hard rock bullet does not use a cutting ring. Cables attached to each bullet (D) help extricate the bullet from the formation after it has been shot.

9 Guns are retrieved to the surface with core bullets attached (E), and technicians remove core samples and place them in bottles. The bottles are labeled, boxed and sent to a laboratory for analysis. (Photograph of retrieved core gun courtesy of Benjamin Wygal.). demonstrate the application of the XL-Rock tool, ing in oil and gas wells, a technique practiced The rst percussion coring tool was intro- a recently introduced large-volume Rotary side- frequently even today, crucial in the initial phase of duced in the Today, all major wireline ser- wall coring service, in an unconventional eld development. vice companies offer percussion coring tools, also resource play in the northeast US and a deepwa- Unfortunately, coring can impact drilling ef - referred to as core guns. Core guns such as the ter well offshore Australia.

10 Ciency because of the time required to cut and Schlumberger CST chronological sample taker recover whole cores. Depending on the coring appear similar to the original percussion coring Getting the Core objective and cost limitations, E&P companies tools introduced almost a century ago; however, The advent of Rotary drilling systems in the 1800s may deem conventional coring nonessential. In improvements to core gun hardware have made possible the practice of whole core drilling. such cases, they may turn to percussion and resulted in the reliable and cost-effective sys- The origin of core drilling is attributed to French Rotary Sidewall coring to gain valuable reservoir tems in use today. civil engineer Rudolph Leschot, who mounted rock information. Of the two methods, percussion Core guns have hollow, barrel-shaped bullets diamonds on a hollow, circular drill bit to acquire coring has been the most common; however, in mounted on a carrier (above).