Prior to developing any unconventional reservoir, an operator must confidently answer two fundamental questions. Firstly, are there sufficient quantities of hydrocarbon present in the formation, and secondly, can these hydrocarbons be extracted economically?
Weatherford Laboratories provides answers to these questions with its wellsite geosciences service, a recent addition to a robust portfolio of formation evaluation techniques. The service combines portable technologies with interpretative expertise to deliver reservoir measurements at the wellsite. These measurements help operators optimize production and reduce develop ment costs in shale plays to ensuring timely delivery of reservoir data in high-pressure/high-temperature (HP/HT) environments.
Sharpening the View in Shales
Over the past decade, unconventional oil and gas plays in North America have flourished, as shale has shifted from being a cap rock for a hydrocarbon-bearing reservoir to a commodity rock in its own right – a viable production target holding vast quantities of oil and gas.
But until recently, operators working in shale plays have resigned themselves to a ‘perf and pray’ mindset, in which they set their packers a certain distance apart, perforated every zone in exactly the same way, and then hoped that the resulting production profile from each zone matches their plan.
Increasingly, geologists understand that the data gleaned from rock cuttings and gas analysis can be used to optimize a well’s completion so that production from each fractured zone is maximized. The ability of the wellsite geosciences service to accurately report on the geochemistry, mineralogy and elemental composition of each zone provides in-depth and near real-time reservoir information that conventional techniques such as core analysis cannot offer.
Conventional core analysis is typically not a cost-effective method in the long horizontal wells that characterize shale plays. For new shale plays, it is not uncommon to core three or four pilot holes, which requires sending these core samples to an offsite laboratory for analysis. Getting all the pertinent reservoir data back from these samples – the data that is important for ensuring that the laterals land to their optimal target depth and location – might take three to four months.
In this case, the operator has two choices: to either immediately drill the lateral without the benefit of this core data and make educated guesses as to where to frac, or to move the rig off location to drill another well, and come back to complete the lateral once the core analysis is available.
By contrast, Weatherford’s new wellsite geosciences service provides a complete report of this reservoir information in 24 hours. Completion engineers can then take this information and identify the ideal spot to land their interval, almost immediately, and formulate the proper fracturing fluid for the formation.
Four Primary Technologies
The wellsite geosciences service incorporates four primary technologies – the proprietary GC-TRACER® surface gas detector, the Source Rock Analyzer (SRA), an X-ray diffraction (XRD) tool and RockWiseSM X-ray fluorescence (XRF) – to enable field analysis of formation gas and rock samples. Unlike core samples, which have to be cut from the formation and analyzed in the laboratory, these gas and rock cuttings samples are a natural byproduct of the drilling process and are therefore essentially free.
The surface gas detector system incorporates extraction technology to obtain and characterize formation gas samples from drilling mud. Taking only approximately 55 seconds, it provides precise compositional analysis of formation gases, including light-end alkanes, benzene, toluene, nitrogen and carbon dioxide. This data is used to elucidate the presence and quantity of hydrocarbons in a reservoir. The system’s capabilities exceed those of traditional gas-agitator-trap systems, which prolong exposure of samples to air and leave substantial amounts of gas in mud, thereby skewing the analysis and providing a less accurate representation of the reservoir.
Also proprietary, the SRA pyrolyzes rock samples to offer insight into a reservoir’s expected hydrocarbon production. Specific measurements include total organic content (TOC); hydrogen and oxygen indices, used to estimate kerogen type and quality; and thermal maturity, used to assess production potential.
Two rock characterization technologies – the portable XRD tool for direct quantification of mineralogy and the XRF tool for direct measurement of elemental composition – help keep the wellbore in zone and locate brittle zones in formations, which is particularly important for shales. As a general rule, the more brittle a zone, the more receptive it is to hydraulic fracturing.
Instruments Designed for Mars
Mineralogical information provided by the XRD helps identify formation tops and variations within the formation, as well as determine the most attractive intervals for lateral drilling and where to fracture for optimal production. The instrument was initially designed by NASA to characterize the mineralogy of rocks on Mars. The space agency launched the XRD technology in November 2011 as part of a Mars Rover mission, and it will start analyzing the Martian landscape in August 2012.
Similarly, the XRF analysis directly measures the elemental composition of cuttings, providing data for up to 30 elements – 12 major and 18 trace elements – with a high degree of precision and within 30 minutes of sample collection. The primary use of this data is for chemostratigraphic correlation to increase confidence of borehole position. XRF-enhanced mineralogical determinations, including estimation of certain clay types and refined brittleness assessment, document subtle changes in rock composition that cannot be detected by other technologies. With this knowledge, drillers can make on-thefly changes in trajectory to land and keep the wellbore in the target zone.
Frac Stages Reduced
An operator in the Eagle Ford shale gas play in Texas deployed the wellsite geosciences service to optimize their hydraulic fracturing program. The realtime formation data afforded by the service allowed the operator to reduce the number of logs run on well by four, which lowered the risk of getting a logging tool stuck in the hole and requiring additional rig and tool time to remedy the problem. And because the service provided the operator with a more accurate picture of which formation zones had the right combination of brittleness and TOC, they were able to eliminate two frac stages per well.
The significance of eliminating stages comes into focus when one considers a sobering statistic. According to a recent industry study, the act of blindly fracturing an entire interval can result in as many as 30 to 40 percent of the perforation clusters contributing only 1 percent of total production. This is all the more important when one considers that it is not uncommon to spend up to US$250,000 on one frac stage, and many shale gas wells may require up to 20 stages.
A 40 percent failure rate may translate to an operator overspending US$2 million on a single well. But by eliminating just one frac stage, the wellsite geosciences service pays for itself.
Data Acquisition with HP/HT
The wellsite geosciences service also offers benefits in high-pressure/high temperature (HP/HT) reservoirs, in which ambient temperatures exceed 325°F (163°C). These conditions are typically too severe for conventional logging tools that have to be deployed downhole. By contrast, an XRF unit deployed at the surface can ensure acquisition of an elemental spectral gamma ray from potassium (K), thorium (Th) and uranium (U), measured from the cuttings. The data can then be used to update seismic models and verify the driller’s location in the stratigraphic sequence.
Since its debut in early 2010, the service has enabled operators in North America, and increasingly in several South American shale plays, to streamline exploration projects, optimize wellbore placement, and gain a new measure of geological confidence as drilling programs proceed.