GEOSTEERING HORIZONTAL WELLS USING HIGH SPEED CHROMATOGRAPHIC GAS RATIOS

Presented at: The Aberdeen Formation Evaluation Society, SPWLA Aberdeen Chapter Conference on
Technical Solutions for Surviving An Erratic Oil Price, November 23 1999

Author: D.P.Hawker, Datalog, 520 - 12ASt SE, Calgary, Alberta, E-mail

Presenter: A. Gittos, Datalog, Unit 1 Cadleigh Close, Lee Mill Ind Est, Plymouth, Devon; Tel 1752-893841, E-mail

Abstract
Presented at: The Aberdeen Formation Evaluation Society, SPWLA Aberdeen Chapter Conference on
Technical Solutions for Surviving An Erratic Oil Price, November 23 1999

Author: D.P.Hawker, Datalog, 520 - 12ASt SE, Calgary, Alberta, E-mail

Presenter: A. Gittos, Datalog, Unit 1 Cadleigh Close, Lee Mill Ind Est, Plymouth, Devon; Tel 1752-893841, E-mail

Introduction
Reservoir productivity can be significantly increased by drilling horizontal wells through zones of optimum reservoir quality but, at the same time, steering the wells with Logging While Drilling (LWD) increases overall drilling costs.

Steering a well requires the identification of lithology changes, reservoir CONTACT USs and reservoir quality. These requirements are met through traditional Mud Logging techniques, evaluating lithology, porosity and fluid type through penetration rate, drilled cuttings, fluorescence and gas responses.

Chromatographic gas ratios provide an extra dimension to geosteering wells, through accurate determination of reservoir fluid changes and CONTACT US points, providing a technique where CONTACT USs are strictly fluid based, with no corresponding lithology changes required for the LWD gamma tool to work.

Requirements
The chromatography must be high speed in order to provide the necessary depth resolution and accurate CONTACT US and zonal definition. The examples shown in this paper are with an analysis time, methane through pentane, of less than 30 seconds. In addition, the gas ratios must be calculated real-time so that reservoir fluids are characterized as drilling proceeds.

The ratios (2,3) used for geosteering are:

Wetness Ratio Wh = [(C2+C3+C4+C5)/(C1+C2+C3+C4+C5)] x 100

Balance Ratio Bh = [(C1+C2)/(C3+C4+C5)]

Character Ratio Ch = (C4+C5)/C3

These were derived, empirically, by comparing wellsite gas data to subsequent test and production data. Although guidelines exist (Table 1) by which to evaluate these ratios, many factors affect the circulation and gas extraction process. Definitive values can therefore vary and if this is not recognized, be misinterpreted.

For example:
• Heavier oils with low C1 can result in low Bh and high Wh.
• In underbalanced drilling, producing zones may release more light end gas and, therefore, a higher Bh.
• Very tight lithologies may lead to formation fluid, especially heavier hydrocarbons, being retained in cuttings porosity and
  not released to the drilling fluid, resulting in lower Wh and higher Bh.
• Even viscous mud systems, with poorer extraction of heavier gases, can produce lower Wh and higher Bh.

Application
Taking these factors into consideration, ratios are better applied at wellsite by evaluating the trends and relationship of the curves (Table 1). The geosteering application is largely based on the relationship between the Wh and Bh curves to identify gas-oil CONTACT USs (Wh and Bh cross over) and oil-water CONTACT USs (Wh and Bh typically separate due to the heavy residual component).

Ch is principally used in initial reservoir evaluation to confirm whether a gas prediction is actually identifying a productive gas phase or an association with light oil. Putting this to the geosteering application, in certain situations where Wh and Bh are close together, generally still indicating gas but perhaps criss-crossing, Ch may be used to determine the gas-oil CONTACT US.

Ratio values, as described, may be higher or lower than expected, but the trends and relationships of the curves remains an extremely viable tool to steer wells. Examples of where ratios may not be definitive include identifying very dry gas CONTACT USs, or identifying oil-water CONTACT USs with certain heavy oils. In these situations, there is insufficient compositional variation for the ratios to identify the CONTACT US.

Knowing the composition, and resultant ratios, through individual zones is therefore an important component to geosteering. Once a vertical ratio profile is known (Figure 1), from a pilot well for example, it can seen whether geosteering the lateral section is viable. Effectively, the ratios can be “turned on their sides” to provide the boundaries and control necessary to steer the well (Figure 2). Should the ratios start to cross, or separate, then it is known that a downward or upward course correction is required. The log examples (Figures 3-5), from Western Canada, show how the zones and CONTACT USs are identified on lateral well sections.

Summary
Existing ratio principles can be used in most reservoirs to steer lateral well sections by properly applying them. Through rapid real-time analysis and calculations and informed evaluation of ratio curve trends, established controls can be followed in order to steer the well and dramatically reduce the overall cost of the drilling program.

Table 1:
Guidelines to fluid characterization using the Wetness, Balance and Character ratios. The ratios are plotted alongside to illustrate how reservoir profiling can be achieved, either while drilling, or from subsequent depth based analysis.

Figure 1:
Typical Wh and Bh ratio profiles, showing an oil-bearing reservoir in A, and a reservoir gas in B

Figure 2:
Applying the ratios on a lateral basis, in order to identify changes and CONTACT USs and steer the well.

Figure 3:
Horizontal log section showing GOC, medium-heavy oil bearing sand, and basal sand/shale transition.

Figure 4:
GOC shown for a light oil bearing sand.

Figure 5:
Oil bearing sand, dipping right-left, showing determination of GOC and OWC.