This adopted report is to evaluate the lithologies in well for the presence of Hydrocarbon with the following job scope; Job design, Equipment preparation, Crew management, Project execution and billing, Basic interpretation.
Introduction
Open hole wireline measurements provide the first and comprehensive data sets for evaluation and analysis needed to correctly determine the lithology, properties, and fluid contents of a well among others. From the results of these analyses, better understanding and conclusions could be drawn on the response of the reservoirs as compared with the pre-drilling data, well trajectory, offset wells and natural basins. The acquired data sets’ first contact is usually with the Geologists and/or Petro physicists who work together with the oil servicing companies to determine configurations, necessities, resolutions and deliverables required.
This project aims to design, prepare, and implement one such data acquisition exposition that will acquire and record resistivity, sonic, caliper, density, porosity, spectral gamma ray and spontaneous potential data to evaluate the lithologies in well for the presence of Hydrocarbon
Scope
Whenever a company decides to go on hydrocarbon exploration, there usually is a lot of uncertainties as to the decisions of which area to explore are only based on surface data and available history which is mostly not enough for a definitive successful venture. Therefore, when they eventually go ahead to drill at a place, they require more real-time measurements and data to make critical decisions. It is at such points that formation evaluation expositions are necessary.
For each of such exposition, there is a clear set of objectives from the Client encompassing the kind of measurements required, the environment to be expected downhole and the quality of data needed upon completion of the acquisition. Once this is understood and agreed upon, then the different applicable combinations of tools and equipment are selected to meet these requirements.
The results gotten from these measurements would grant a better understanding of the well; if planned reservoirs were captured while drilling, quick look on the presence of hydrocarbon or not, condition and rugosity of the drilled well, need to drill further, sidetrack or plug and abandon, as well as further qualitative derivations.
For the project in view, tools from the Petro-physics family which could independently or in combination provide resistivity, sonic, caliper, density, porosity, spectral gamma ray and spontaneous potential measurements while still meeting the demands of resolution and logging environment were selected. Within the logging world, such combination could be referred to as Super-combo or Quad-combo+.
This tool string is often deployed first (as was the case in this project), especially in a scenario where there are several other measurements required. It acquires most of the basic petrophysical and lithological qualities of the well and formations as well as giving the borehole width, which is an important indicator of borehole and log quality.
The porosity, density, and resistivity logs (also known as the triple-combo logs) in addition to their nomenclature provide permeability, saturation and invasion profiles and are an absolute necessity for any form of interpretation. The sonic measurement takes it a step further by giving compressional and shear slowness properties of the different reservoirs. The gamma ray is a very defining property of the well as it rarely changes or is affected by subsequent operations on the well as such is very important for correlation. A spectral gamma ray is a more comprehensive log which gives the compositions of gamma ray from thorium, potassium, and uranium, very useful in understanding deposits. Spontaneous potential and caliper measurements give the difference in downhole potential versus a known surface potential and hole width/rugosity, respectively.
Project Methodology
Design
The first part of the project’s design was to acquire the relevant well data from the Client to help prepare the correct equipment. These data including but not limited to.
- The borehole data (bit size, total depth, and casing inner diameter)
- The maximum expected temperature and pressure
- The maximum planned deviation, and doglegs
- The formation salinity and matrix to be used
- The fluid data (Water-based mud, oil-based mud, brine, or freshwater)
- The mud properties (density, salinity, losses, the composition of additives like barite, K, Al, etc.)
Armed with the well data, the next design phase of selecting the appropriate equipment commenced. In this phase, simulations were done to determine combinability of tools, telemetry requirements, compatibility with electrical logging cables, surface acquisition requirements, required force/pulling capacity, conveyance suitability, optimal logging parameters, contingency planning, crew competencies and safety matrix of the operation.
Figure 2.1: Force/pulling capacity simulation results
The results of some simulations are so complex that subject matter experts (SME) will have to be contacted. On this project, tool combinability and telemetry issues were complex matters, and restrictions of setup for maximum pull was another concern at the design phase. This led to the next phase of design which was the logging while on paper (LWOP). This is typically a meeting or maybe series of meetings with all stakeholders to have discussions about the operation and highlight any challenges faced and anticipated, results of simulations, logging modes (standard, high or extra-high resolutions) and logging intervals.
- Prepare
Several tools make up the Petro-physics family of acquisition tools and based on requirement and/or environment, could be combined in several possible ways. The combination used for this project included the following:
- Spontaneous Potential Adapter (SPA)
- Auxiliary Compression/Tension Sub (ACTS)
- Enhanced Digital Telemetry Cartridge (EDTC-B)
- Hostile Environment Natural Gamma Ray Sonde (HNGS)
- Highly Integrated Gamma-Neutron Sonde (HGNS)
- Three Lithology Detector (TLD)
- Global Positioning Inclinometry Tool (GPIT)
- Powered Positioning Cartridge (PPC)
- Multi Array Sonic Tool (MAST)
- High-Resolution Laterolog Array (HRLA)
EDTC-B
EDTC-B is a downhole tool that combines two commonly run sensors with a high- speed telemetry downhole modem that can be used in high-pressure and high- temperature environments. The primary function is to provide high-speed (>1 Mbps) communications between the wireline tools downhole and the acquisition system at surface. Additionally, it includes a scintillation gamma ray detector that also measures formation gamma rays. The gamma ray log is generally used to depth match between logging runs.
HNGS
The HNGS utilizes two bismuth germanate (BGO) scintillation detectors to measure the natural gamma ray radiation of the formation. Depending on the environment, the HNGS may be run on other tool strings as well. The gamma ray log is generally used to depth match between logging runs.
HGNS
The HGNS uses a chemical neutron source and two detectors to measure porosity. It has an accelerometer that detects speed and well orientation, and it also detects naturally occurring gamma rays which are used to serve as a correlation for future logs.
TLD
The TLD (also known was HDRS) provides density, density porosity, invaded zone resistivity, formation photoelectric factor, and caliper information that gives information about the shape and quality of the whole. It is made up of a skid and a communication cartridge and is a new-generation tool that is designed to replace the HLDS tool.
GPIT
The GPIT provides inclinometer measurements. Tool orientation is defined by three parameters: tool deviation, tool azimuth and relative bearing. The GPIT tool uses both a three-axis inclinometer and a three-axis magnetometer to make measurements for determining these parameters.
PPC
PPC is a four (4) armed caliper tool that can be used as centralization or an eccentralization equipment. There are motors in the tool that help to apply force on the arms based on the setup and this measures the borehole width when opened. For this project, it was used as both centralization and a four-axis caliper tool.
MAST
The MAST tool measures compressional and shear slowness with multiple boreholes compensated monopole and crossed-dipole transducers. The radial and axial measurements of the stress-dependent properties of the rock are the basis of a comprehensive geomechanical characterization or stresses induced from the drilling process.
HRLA
The HRLA tool provides six resistivity measurements with different depths of investigation (including the borehole/mud resistivity, and five measurements of formation resistivity with increasing penetration into the formation). The HRLA is typically deployed in low mud-resistivity (water-based mud) environments.
The exact combination of the tool string from top to bottom was LEH-SPACER-AH369-SPA- SAH-ACTS-EDTC-HNGS-HGNS-TLD-(KJ-KJ)-GPIT-SPACER-PPC-MAST-PPC-(KJ-KJ)-HRLA-(KJ-KJ)-SPACER-BNS. Knuckle joints (KJ) placed at strategic points in the string allow for flexibility and reduce the effective rigid length of the tool. Spacers used to allow for fishing contingency, AH369 as electrical isolation for the HRLA, Swivels (SAH) included releasing torque to the cable while Bottom nose (BNS) is just a bottom electrical and fluid plug. Also, the SPA (Spontaneous Potential adapter) directly measures the difference in potential between a known surface reference and downhole, ACTS (Auxiliary Compression/Tension Sub) provides downhole tension/compression information and LEH (Logging Equipment Head) allows the tools to connect and interface with the wireline vis-à- vis surface acquisition system.
During the prepare phase, the planned tool string was physically connected, and a logging operation was simulated with derived parameters and tool response closely monitored. The aim is to spot any potential issues and check tool functionality in similar environments. If any issues are discovered, tools had to be taken to the maintenance laboratories for a fix.
Asides from the tools, accessories that are associated with the tools had to be sorted out and checked optimally. The assigned acquisition unit was also prepared including ensuring the IP (Internet protocol) from all systems and units were properly set up to the network drawer/router and communicating as they should, Continuity and Insulation checks were also done for the cable and terminations. With equipment out of the way, crew competency had to be checked as well against the planned services, job profile and logging environment. This is used to select the best possible crew to go and perform the job while bearing in mind that the safety of everyone is the ultimate objective.
Figure 2.2: Super-combo tool string
- Implementation and results
Upon arriving at the job location, the first action is usually to discuss the program with the company representative at the location, more like ensuring all the ‘I’s are dotted and ‘T’s crossed. After this, the surface equipment is positioned, set up and tested in readiness for the acquisition proper. When the rig is ready, the tool string is picked up one after the other to the rig floor and lowered into the well; at this point, it is critical for the supervisor to ensure the right tools are picked at the right time. Simultaneously, the surface setup must be finalized for acquisition, the setting of parameters, entering the proper well properties, and confirming no issues with communication.
Figure 2.3: How tool telemetry works
The tool string is then run carefully to the bottom of the borehole using the wireline winch. Commands are sent from the acquisition system through the wireline cable to the tools and received through the same route in a reverse manner. During the descent, critical pointers need to be monitored to ensure the string is moving smoothly downhole and no restrictions. At the bottom, all calipers are opened systematically by controls on the surface, and the tool is set to record data coming up over the client’s agreed interval of interest. For logs like in this project, a repeat log is usually required over any 200ft interval of the Client’s choosing as a form of log quality control to ensure the tools are reading the same thing as previously recorded. Some examples of typical deliverables from this tool string are shown below.
Figure 2.4: Sample Spectral gamma ray log
Figure 2.5: Sample triple combo log with SP
Figure 2.6: Sample sonic log
A qualitative analysis of the well and formations properties is then possible by studying the curves on each of the log and their relationships with each other. For example, when the GR curve reads low, resistivity reads low with the curves separated from each other, and the density
and porosity curves overlay, this would suggest permeable water-bearing sand. These logs thus allow geologists and Petro-physicists to identify reservoirs and plan for subsequent actions on the well be it further exploration or development or plug and abandoning of the oil well.
Conclusion
The benefits of a formation evaluation exposition cannot be overemphasized. For this project, the steps taken in designing, preparing, and executing one such exposition was discussed and some of the typical outcomes presented. The data acquired included resistivity, sonic, caliper, density, porosity, spectral gamma ray and spontaneous potential measurements. As much as this may sound very petroleum, the actual preparation and execution involved a lot of understanding of the electrical and electronics principles. From the internal workings of the tools to how it accurately transmits data from downhole to surface and then interpreted into sets that make sense to the end user, these all relate to the instrumentation, power and communications aspects of the electrical field.
Recommendations
This project recommends that petrophysics logs should always be deployed as the first set of data acquisition for any drilled well. The value of these data sets has been proven to be priceless in any exploration or development venture.
It also recommends that any person put in the position of a supervisor should have clear understandings of electrical and electronic theories as there are times issues arise during execution and it is this knowledge that will help the person to successfully troubleshoot and solve the problem to complete such time-sensitive projects.
Contributor: Mkpouto Umana Okoko (adopted report)
