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1 Chemostrat Ltd., Unit 1 Ravenscroft Court, Buttington Cross Enterprise Park, Welshpool, Powys, SY21 8SL, UK kenratcliffe@chemostrat.com

2 Chemostrat Australia, Suite 17, 44 Kings park Road, West Perth, WA 6005

3 Apache Corporation, 700 9th Avenue SW, Calgary, Alberta, Canada

Determining well-bore pathways during multilateral drilling campaigns in shale resource plays: an example using chemostratigraphy from the Horn River Formation, British Columbia, Canada

K.T. Ratcliffe1, J. Woods2, C. Rice3

Keywords: chemostratigraphy, shale resources, Horn River Formation, NE British Colombia

SummaryThis paper demonstrates how the technique of

chemostratigraphy is used to define a regional stratigraphic framework for the Horn River Formation, a shale resource play in north eastern British Colombia, Canada. Initially, a regional stratigraphic framework is defined and subsequently the technique is used in multilateral wells drilled from a central pad. The latter enables the well-bore pathway in to be placed relative to the stratigraphy of the pilot hole.

IntroductionThe Horn River Formation in north east British Colombia is

estimated to have 500 Tcf gas in place, which makes it the third largest North American natural gas accumulation discovered prior to 2010. Lithologically, it is composed of dark grey to black, variably organic rich, siliceous and calcareous mudrocks. In the area of this study it is approximately 200m thick and is further subdivided into the Evie, Otter Park, and Muskwa members (Figure 1). The Evie and Muskwa members are notably organic-rich and siliceous.

Typically, when exploiting the Horn River Formation, a series of lateral wells are drilled from a single central pad. In all such drilling campaigns, knowing where the well-bore pathway travelled relative to the regional stratigraphy and being able to relate the well-bore pathway back to the pilot hole is imperative (Schmidt et al., 2010). However, hitting and remaining in the “sweet spot” when drilling in shale resource plays is particularly challenging. A recent study conducted by Halliburton concluded

Figure 1. Location of Horn River Basin and generalised stratigraphy of the basin. Red box in the stratigraphy column highlights study interval.

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Results

Regional chemostratigraphy

The elements and element ratios plotted as chemical logs for two vertical study wells in Figure 2 allow the study interval to be divided into 5 chemostratigraphic packages and 9 geochemical units, the main features of which are:

Keg River Carbonates: this formation underlies the Horn River Formation and is geochemically distinctive due to is high CaO values, reflecting its clean limestone lithology.

Package 1 is characterized by high values of EFV and is also differentiated from the overlying package by its low RTi and high SiO2/Zr values. This package is approximately equivalent to the Evie Member.

Package 2 is differentiated from those below and above by its high RTi and low EFV values. It also typically has upward increasing Th/U values. This package is approximately equivalent to the Otter Park Member.

Package 3 is differentiated from the packages above and below by its low Th/U and high EFV and SiO2/Zr values.

that “approximately 50% of wells geosteered using the conventional gamma ray geosteering methods within an area of the Haynesville were misplaced for more than 50% of their lateral length.” (http://www.epmag.com/Production-drilling/Geosteering-Unconventional-Shale-Reservoirs-Potential_80771). Here, the technique of chemostratigraphy is applied to a series of lateral wells to demonstrate how the technique can help improve time spent in a given zone.

MethodologyChemostratigraphy involves the characterization and

correlation of sediments using variations in their elemental compositions (Ratcliffe et al., 2010 and references cited therein). It is widely applied to help with stratigraphic correlations in petroleum provinces (Pearce et al., 2005a, Ratcliffe et al, 2006, Hildred et al., 2010, Wright et al., 2010). Elemental data have been acquired here using Inductively Coupled Plasma Optical Emission Spectrometry and Mass spectrometry (ICP OES MS) (Jarvis and Jarvis 1995), which results in data for 50 elements being acquired; 10 major elements, 26 trace elements and 14 rare earth elements.

The dataset acquired from the Horn River Formation for this study to date is from 4 pad locations, with data from 4 vertical holes and over 30 lateral wells. Total sample numbers exceed 1000 and all data have been acquired from cuttings samples.

Figure 2. Chemostratigraphic characterisation and correlation of two vertical wells. Package 1 equates approximately to the Evie Member, Package 2 to the Otter Park Member and the Package 4/Package 5 boundary correlates approximately to the top of the Muskwa Member. Refer to Figure 1 for lithostratigraphic nomenclature.

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dETERMINING WELL-BORE PATHWAYS dURING MULTILATERAL dRILLING CAMPAIGNS IN SHALE RESOURCE PLAYS: AN ExAMPLE USING CHEMOSTRATIGRAPHY FROM THE HORN RIVER FORMATION, BRITISH COLUMBIA, CANAdA

Package 4 is somewhat transitional between Package 3 and Package 5, with upward increasing RTi and Th/U values. Packages 3 and 4 are approximately equivalent to the Muskwa Member.

Package 5 is typically poorly samples since it is the top of the zone of interest. It has the highest RTi values of the study interval.

By comparing these data with mineralogy and TOC data and by using multivariate statistical analysis and graphical plots in the manner described in Pearce et al., (2005b), Svendsen et al., (2007), Ellwood et al., (2008) and Pe-Piper et al., (2008) it is possible to make the following interpretations of the controlling factors of the key element ratios used in Figure 2.

CaO: directly proportional to the amount of calcite in the samples.

U: directly proportional to the amount of preserved organic matter in the samples.

RTi: Calculated by summing Al2O3, TiO2, Fe2O3 and K2O values provides an indication of the amount of terrigenous material in the sediment.

Th/U: Here this ratio is modeling the amount or clay versus organic content.

EFV: Calculated using EFVsample = V/Al2O3sample / V/Al2O3standard shale. Tribovillard et al., (2006) demonstrated that where EFV =1 the depositional environment was oxic. High values of EFV indicate that the depositional environment was dysoxic, anoxic or euxinic.

SiO2/Zr: This ratio highlighted areas of the sequence that contain biogenic silica, which has been demonstrated by others (Wright et al., 2010b; Ratcliffe et al., 2012).

Based on the interpretations of elements above, it can be concluded that during deposition of Packages 1 and 3 the depositional environment was anoxic, terrigenous input was low and that a high portion of the quartz being deposited was biogenic in origin. The higher CaO values in Package 1 than Package 3 (Figure 2) could indicate a shallower depositional environment (above the CCd ?) or the carbonate could be in the form of detrital carbonate material. Packages 2 and 4 were both deposited in relatively oxic conditions, with increasing amounts of terrigenous material entering the system. The Th/U spikes in Package 2 are somewhat enigmatic since they do not have a corresponding increase in RTi or decrease in U. They probably indicate a distinctive composition of terrigenous material (volcanogenic ?) was supplied to basin toward the top of Package 2.

Placement of lateral wells

Figures 3 and 4 show the interpretation of two lateral wells from the d-52-L/094-O-08 pad, one drilled south east from the pad, the other northwest from the pad. The key to interpreting well-bore pathways in highly deviated wells is a) having knowledge of the regional dips and b) being able to accurately determine where the heel of the well was relative to the stratigraphy. The dip of beds can be determined from the chemostratigraphy, where a unit boundary is crossed more than once in a well, for example in Figure 3, the top of Unit 2.1 is crossed at least twice, joining those points indicates the direction of bedding dip relative to the

well-bore. Typically, however, the regional bedding dip is already known prior to drilling and can be accounted for while interpreting the lateral pathways.

Figure 3 displays the vertical, build and lateral section of well d-F52-L/094-O-08. In the build section, the Package 4 / Package 3 and Package 3 / Package 2 boundaries are clearly recognizable by comparing the changing values of RTi, SiO2/Zr, EFV and Th/U in this well with the wells displayed on Figure 2. Between 2755-2760mTVD in the build section, the high Th/U spike that defines Unit 2.3 is seen, indicating that the heel of this well lies within Unit 2.2 or 2.1. In the lateral section from 2900-3620mMd the Th/U values are sufficiently low to indicate that this section of the well is within Unit 2.1. At 3620mMd, the Th/U increases to levels more typical of Unit 2.2, indicating that the well-bore was travelling up stratigraphy. From 3620 – 4600mMd, the chemistry is relatively consistent and typical of Unit 2.2, apart from the sample at 4500mMd which has high CaO values. This sample almost certainly represents a calcite stringer that was not seen in vertical sections due to sample resolution; when drilling at low angle to bedding, even stringers a few centimeters thick are likely to be resolved and these can provide invaluable information on more indurate layers that will affect subsequent fraccing. At 4600mMd, where the well-bore starts to increase TVd notably, the Th/U values decrease indicating that this drop in TVd within the wellbore resulted in it re-entering Unit 2.1. The toe of the well remains within Unit 2.1.

Well d-O52-L/094-O-08 is on the opposite side to the central pad to well d-F52-L/094-O-08 (Figures 3 and 4). Therefore, while the lateral section in well d-F52-L/094-O-08 had to “chase” bedding up-dip, the lateral section in well d-O52-L/094-O-08 is “chasing” the bedding down-dip. In well d-O52-L/094-O-08, the Package 4 / Package 3 and Package 3 / Package 2 boundaries are clearly recognizable by comparing the changing values of RTi, SiO2/Zr, EFV and Th/U in this well with the wells displayed on Figure 2. The high Th/U values at 2760mTVd in the build section indicate that the heel of the well lies within Unit 2.3. Th/U values remain in the lateral section until 3250mMd when they drop, indicating that the well penetrated Unit 2.2 at that depth. At 3550mMd, the Th/U increases in association with a local TVd maximum in the well-bore, suggesting that the Unit 2.2 / 2.1 boundary was penetrated and the well-bore was within Unit 2.1 between 3550-3800mMd. At 3800mMd, a slight decrease in TVd of the well-bore results in return to a Unit 2.2 geochemical signature. At 4550mMd, the well-bore rapidly starts to increase TVd which initially results in penetration of Unit 2.1, before the toe of the well T.d.’s in Package 1, as indicated by the sharp increase in EFV values in the last two samples analysed.

ConclusionsBy using chemostratigraphy, regional correlations of shale

reservoirs can be achieved, which help understand broad, basinal stratigraphies and events. However, a potentially more important application is the ability to use changes in elemental compositions to determine well-bore pathways in multilateral drilling campaigns. The data presented here have been acquired in a laboratory, but they can be largely acquired while drilling at wellsite within, c. 20 minutes of a cuttings sample arriving at surface. Therefore, this application of chemostratigraphy has large potential in real-time geosteering, not only in the Horn River Basin, but in any shale resource play of any age from around the World.

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Figure 3. Chemostratigraphic interpretation of the build and lateral section of well d-F52-L/094-O-08. Lower left inset shows the well-bore pathways in plan view, with the well in this figure highlighted in red.

Eastern Australasian Basins Symposium IVBrisbane, QLD, 10–14 September, 2012 147

dETERMINING WELL-BORE PATHWAYS dURING MULTILATERAL dRILLING CAMPAIGNS IN SHALE RESOURCE PLAYS: AN ExAMPLE USING CHEMOSTRATIGRAPHY FROM THE HORN RIVER FORMATION, BRITISH COLUMBIA, CANAdA

Figure 4. Chemostratigraphic interpretation of the build and lateral section of well d-O52-L/094-O-08. Lower left inset shows the well-bore pathways in plan view, with the well in this figure highlighted in red.

AcknowledgementsThe authors would like to thank Apache Corporation for their

permission to display this data in the public domain, and are grateful to Chemostrat for allowing the time and providing the support needed to prepare the presentation.

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