Well Logging Logging
Introduction (Chapter I) By Dr. Eng. Muslim
A. History, History, Present and Future of A Science Science and Technology echnology
• The fundamental technology of electrical coring or electro log was first done in 1927 where Schlumberger brothers made the first electrical resistivity measurement in an oil well near Pechelbronn/Alsace, a location in Paris.
• The design of the technology is still valid in our days : 1.Create a continuous plot of a measured property (resistivity) as a function of the depth (measured) and call it “the log.” 2.With the fundamental advantage of representing continuous information thus log must be transformed into information for reservoir characterization (porosity, saturation) by a process called interpretation.
• To understand the quantitive interpretation petrophysical knowledge is necessary. The first step was done by Archie’s famous equation (Archie 1942). 1
Ro n Sw Rt
Where Sw is water saturation, Rt is the spesific electrical resistivity of the formation, Ro is the specific electrical resistivity of the formation at water saturation = 100% and n is Archie’s empirical saturation exponent.
• A formation or reservoir characterization cannot be derived from one type of logs alone – it needs a combination of various physical parameters in order to derive a consistent model of the formation (complex interpretation). The historical development of borehole geophysics is therefore characterized by the development of various systems with defined sensitivity. Cornerstones of the first period are: 1. Resistivity logs (first commercial logs), directed at water saturation determination for clean rocks, 2. Spontaneous Potential log, directed at the separation of sand (clean rock, reservoir) and shale,3. Acoustic log, the only “porosity log” based on Wyllie’s equation (Wyllie et al. 1956) in the early days
B. Fundamental Problems And The Way We Go
•
Figure 1-1 defines the fundamental questions with regard to locality, depth and geometry of the reservoir – in most cases this general model is derived from geological, sedimentological and structural studies implementing seismic and other surface geophysical results.
C. Terminology
• In this section, it will make things simpler if we first go through some of the basic terms that will be used throughout this section. 1.STOIIP = stock tank oil initially in place; usually measured in stock tank barrels (stb) 2.GIIP = gas initially in place; usually measured in billion standard cubic feet (bcf) 3.GBV = gross bulk volume 4.NPV = net pore volume
5.HCPV = Hydrocarbon pore volume
6. Φ = Porosity
7. K = permeability 8. Sw = water saturation
9. Sh = Hydrocarbon saturation 10.FWL = free water level
11. HWC = Hydrocarbon/water contact 12. GOC = Gas Oil Contact 13. Gross Thickness = Total Thickness of the formation as encountered in a particular well 14. Net Thickness = part of the gross thickness that contains porous rock subject to given cutoff criteria 15. Pay Thickness = The part of the net thickness that is considered to be capable of producing hydrocarbons in a particular well
D. How is logging viewed by others
E. Wellbore Environment
• The Borehole Environment in which logging measurement are made, is of some interest from the standpoint of logging tool designs and the operating limitations placed upon thme. Furthermore, it is important in terms of the disturbance it causes in the surrounding formation in which properties are being measured.
F. Logging Contract
Typically, an oil company will set up logging
contracts with one or more contractors for the provision of logging services. Usually some kind
of tendering process is used to ensure competitive bidding among various companies able to provide such services. Elements that exist in common contracts include the following :
1.
Depth Charge : This relates to the deepest depth that a particular tool will be run in the hole.
2.
Survey charge : This relates to the interval that a particular tool is actually logged in the hole.
3.
Station charge : For tools such as formation pressure sampling tools and side wall samples, this is a charger per station measurement. Usually the contract will make certain specifications regarding when such a charge may be dropped (e.g, if no useful are recovered)
4.
Tool rental : Usually a daily charge for the tools to be on the rig on standby prior to or during a logging job.
5.
Logging unit rental charge : Usually a monthly charge for the logging unit (winch, tool shed, and computers) while it is on the rig.
6.
Base rental : There may be a monthly charge to have a pool of tools available for a client. For LWD tools, this may supersede the tool rental, depth and survey charges.
7.
Engineer charge : Usually a day rate for any engineers, specialists, or assistants present for the logging job.
8.
In – hole charge : Some LWD contracts specify an hourly charge while tools are actually being run in the hole.
9.
Lost – in – hole : For replacement of any tools in the hole during operations. Some contractors provide insurance to the oil companies for a fixed sum per job to identify them against lost – in – hole charges.
10. Cable splice charge : Where tools become stuck in the hole and it is necessary to cut the cable, a charge is usually made for such splicing. 11. Processing charges : Where data require postprocessing (e.g. interpretation of image data or waveform sonic), charges are usually applied in a similar way to survey charges. 12. Data charges : Provision of additional copies of log prints and/or tapes, or data storage, may incur additional charges. 13. Real-time data transmission charges : The oil company will usually be given the option to have data transmitted directly from the wellsite to their office, either as digital data in Log ASCII Standard or binary format or as a print image.
G. Preparing Logging Program
In general, first to be prepared is 1. Particularly during the exploration phase, data have a high value, since (because) they will be used to quantify the reserves and influence the whole development strategy. 2. Particularly in tail-end production, where much of the log data will not even be used for updating the static model, since it is influenced by depletion effects, the value of data becomes much less.
• For a well proposal, a typical program might look something like this :
• For a well proposal, a typical program might look something like this :
H. Wellsite mud logging
The mud log may be of great use to the petrophysicist and geologist in operational decision making and evaluation. Areas in which the mud log may be particularly important include : 1. Identification of the lithology and formation type being drilled. 2. Identification of porous/permeable zones 3. Picking of coring, casing, or final drilling depths 4. Confirmation of hydrocarbons being encountered and whether they are oil or gas.
A. Cutting Descriptions. The mud-logging unit will generally take a sample of the cuttings received over the shale shakers. Some of these samples are placed into sealed polythene bags as “wet samples” and retained. Other samples are washed, dried, and retained as “dry samples.” Washed samples are examined under a microscope in the mud-logging unit and a description made that may be communicated to the office.
• Hydrocarbons may be detected with one of the following methods : 1.Natural Fluorescene Examining the cuttings under UV light may indicate the presence of oil, since oil will fluoresce.
2.Solvent Cut To measure the solvent cut, about 3 cm of dried and crushed sample is placed in a test tube and solvent is added to about 1 cm above the sample.
