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Published on March 9, 2014

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Well Logging Course (2nd Ed.)

1. 2. 3. 4. 5. 6. Well logging introduction Wireline logging Logging consideration MWD vs. LWD Properties of reservoir and logging role Measurement techniques

Recent decades changes in petroleum industry which affected Well logging Changes in petroleum industry hydrocarbons have become increasingly harder to locate, quantify, and produce. In addition, new techniques of drilling high deviation or horizontal wells have engendered a whole new family of measurement devices incorporated into the drilling string that may be used routinely or in situations where access by traditional “wireline” instruments is difficult or impossible. Spring14 H. AlamiNia Well Logging Course (2nd Ed.) 4

well logging meaning The French translation of the term well logging is carottage ´electrique,  literally “electrical coring,” a fairly exact description of this geophysical prospecting technique when it was invented in 1927.  A less literal translation might be “a record of characteristics of rock formations traversed by a measurement device in the well bore.” However, well logging means different things to different people.  For a geologist,  it is primarily a mapping technique for exploring the subsurface.  For a petrophysicist,  it is a means to evaluate the hydrocarbon production potential of a reservoir.  For a geophysicist,  it is a source of complementary data for surface seismic analysis.  For a reservoir engineer,  it may simply supply values for use in a simulator. Spring14 H. AlamiNia Well Logging Course (2nd Ed.) 5

well logging application The initial uses of well logging were for correlating similar patterns of electrical conductivity from one well to another, sometimes over large distances. As the measuring techniques improved and multiplied, applications began to be directed to the quantitative evaluation of hydrocarbon-bearing formations. Much of the following text is directed toward the understanding of the measurement devices and interpretation techniques developed for this type of formation evaluation. Spring14 H. AlamiNia Well Logging Course (2nd Ed.) 6

Well logging scope well logging grew from the specific need of the petroleum industry to evaluate hydrocarbon accumulations New measurements useful for subsurface mapping have evolved which have applications for structural mapping, reservoir description, and sedimentological identification. Identification of fractures the formation mineralogy. well logging is seen to require the synthesis of a number of diverse physical sciences: physics, chemistry, electrochemistry, geochemistry, acoustics, and geology Spring14 H. AlamiNia Well Logging Course (2nd Ed.) 7

Well logging history birth of logging September 5, 1927 By H. Doll and the Schlumberger brothers (and a few others) a semicontinuous resistivity measurement in an old field in Alsace Using a rudimentary device (a sonde) Connecting the device to the surface was a cable/wire • Wireline refers to the armored cable by which the measuring devices are lowered and retrieved from the well and, by a number of shielded insulated wires in the interior of the cable, provide for the electrical power of the device and a means for the transmission of data to the surface. More recently, the devices have been encapsulated in a drill collar, and the transmission effected through the mud column. This procedure is known as logging while drilling (LWD). Spring14 H. AlamiNia Well Logging Course (2nd Ed.) 8

Wireline Logging measurement devices (Sonde) The process of logging involves a number of elements. primary interest is the measurement device, or sonde. Currently, over fifty different types of these logging tools exist in order to meet various information needs and functions. Some of them are passive measurement devices; others exert some influence on the formation being traversed. Their measurements are transmitted to the surface by means of the wire line. Spring14 H. AlamiNia Well Logging Course (2nd Ed.) 10

Well logging Operation The elements of well logging: a measurement sonde in a borehole, the wireline, and a mobile laboratory Courtesy of Schlumberger Spring14 H. AlamiNia Well Logging Course (2nd Ed.) 11

Sonde dimensions Superficially, they all resemble one another. They are generally cylindrical devices with an outside diameter on the order of 4 in. or less; this is to accommodate operation in boreholes as small as 6 in. in diameter. Their length varies depending on the sensor array used and the complexity of associated electronics required. It is possible to connect a number of devices concurrently, forming tool strings as long as 100 ft [30.5 m]. Spring14 H. AlamiNia Well Logging Course (2nd Ed.) 12

Sonde types Some sondes are designed to be operated in a centralized position in the borehole. This operation is achieved by the use of bow-springs attached to the exterior, or by more sophisticated hydraulically actuated “arms.” Some measurements require that the sensor package (in this case called a pad) be in intimate contact with the formation. This is also achieved by the use of a hydraulically actuated back-up arm. Spring14 H. AlamiNia Well Logging Course (2nd Ed.) 13

Sample sondes Next slide illustrates the measurement portion of four different sondes. On the right is an example of a centralized device which uses four actuated arms. There is a measurement pad at the extremity of each arm. Second from the right is a more sophisticated pad device, showing the actuated back-up arm in its fully extended position. Third from the right is an example of a tool which is generally kept centered in the borehole by external bow-springs, which are not shown in the photo. The tool on the left is similar to the first device but has an additional sensor pad • which is kept in close contact with the formation being measured. Spring14 H. AlamiNia Well Logging Course (2nd Ed.) 14

Examples of four logging tools  The dipmeter [on the left]  has sensors on four actuated arms,  which are shown in their fully extended position.  Attached to the bottom of one of its four arms is an additional electrode array embedded in a rubber “pad.”  a sonic logging tool [2nd from left]  characterized by a slotted housing  a density device [3rd from left]  with its hydraulically activated back-up arm fully extended  another version of a dipmeter [on the extreme right ]  with multiple electrodes on each pad. Courtesy of Schlumberger Spring14 H. AlamiNia Well Logging Course (2nd Ed.) 15

The truck These specially designed instruments, which are sensitive to one or more formation parameters of interest, are lowered into a borehole by a surface instrumentation truck. This mobile laboratory provides the downhole power to the instrument package. It provides the cable and winch for the lowering and raising of the sonde, and is equipped with computers for data processing, interpretation of measurements, and permanent storage of the data. Spring14 H. AlamiNia Well Logging Course (2nd Ed.) 16

Measurement speed Most of the measurements are continuous measurements. They are made as the tool is slowly raised toward the surface. The actual logging speeds vary depending on the nature of the device. Measurements which are subject to statistical precision errors or require mechanical contact between sensor and formation tend to be run more slowly, between 600 ft [183 m] and 1,800 ft/h [549 m/h] newer tools run as fast as 3,600 ft/h [1097 m/h] Some acoustic and electrical devices can be withdrawn from the well, while recording their measurements, at much greater speeds. Spring14 H. AlamiNia Well Logging Course (2nd Ed.) 18

vertical resolution The traditional sampling provides one averaged measurement for every 6 in. [15 cm] of tool travel. For some devices that have good vertical resolution, the sampling interval is 1.2 in. [3 cm] There are special devices with geological applications (such as the determination of depositional environment) which have a much smaller vertical resolution; their data are sampled so as to resolve details on the scale of millimeters. Spring14 H. AlamiNia Well Logging Course (2nd Ed.) 19

logging vs. cores, side-wall samples, and cuttings logging is an alternate or supplement to the analysis of cores, side-wall samples, and cuttings Coring  takes time, so expensive  In soft and friable rocks,  only possible to recover part of the interval cored Side-wall cores  obtained from another phase of wireline operations  possibility of sampling at discrete depths after drilling  Side-wall cores disadvantages:  returning small sample sizes, the problem of discontinuous sampling Cuttings,  extracted from the drilling mud return,  are one of the largest sources of subsurface sampling. However, the reconstitution of the lithological sequence from cuttings is imprecise due to the problem of associating a depth with any given sample. Spring14 H. AlamiNia Well Logging Course (2nd Ed.) 20

Well log advantages Although well logging techniques (with the exception of side-wall sampling) do not give direct access to the physical rock specimens, they do, through indirect means, supplement the knowledge gained from the three preceding techniques [Coring, Side-wall cores and Cuttings]. Well logs provide continuous, in situ measurements of parameters related to porosity, lithology, presence of hydrocarbons, and other rock properties of interest. Spring14 H. AlamiNia Well Logging Course (2nd Ed.) 21

measurement while drilling (MWD) To assist drillers in the complex task of a rotary drilling operation, a number of types of information like the downhole weight on bit and the downhole torque at bit are desirable in real time. To respond to this need, a type of service known as measurement while drilling (MWD) began to develop in the late 1970s. A typical MWD system consisted of a downhole sensor unit close to the drill bit, a power source, a telemetry system, and equipment on the surface to receive and display data. Spring14 H. AlamiNia Well Logging Course (2nd Ed.) 24

measurement while drilling (Cont.) The telemetry system was often a mud pulse system that used coded mud pressure pulses to transmit (at a very slow rate of a few bits per second) the measurements from the downhole subassembly. The power source was a combination of a generating turbine, deriving its power from the mud flow, and batteries. The measurement subassembly evolved in complexity from measurements of the weight and torque on bit to include the borehole pressure and temperature, mud flow rate, a natural gamma ray (GR) measurement, and a rudimentary resistivity measurement. Spring14 H. AlamiNia Well Logging Course (2nd Ed.) 25

Logging while drilling The LWD tools are all built into heavy thick-walled drill collars.  Thus, like the wireline tools all the LWD resemble one another. In next slide one particular version is shown that contains several sensors.  The sensors are built into the wall of the drill collar with some protrusions.  However, an adequate channel is provided to accommodate the mud flow.  the device can be run either “slick” or with an attached clamped-on external “stabilizer.”  This latter device centralizes the drill collar and its contained sensors.  When the unit is run in the “slick” mode it can, in the case of a horizontal well, certainly ride on the bottom of the hole.  an interesting feature of LWD  As the drill collar is rotated, data can be acquired from multiple azimuths around the borehole, something not often achievable with a wireline. Spring14 H. AlamiNia Well Logging Course (2nd Ed.) 26

An LWD device An LWD device containing a neutron and density measurement. The panel on the left shows the tool with clamp-on wear bands so that the diameter is close to that of the drill bit. In the right panel the tool is shown in the “slick” mode. Courtesy of Schlumberger Spring14 H. AlamiNia Well Logging Course (2nd Ed.) 27

difference between LWD and wireline logging Diameter Size Unlike wireline tools that are generally of a standard diameter, many of the LWD tools come in families of sizes (e.g., 4, 6, and 8 in.). This is to accommodate popular drilling bit sizes and collar sizes since the LWD device must conform to the drilling string. Another difference between LWD and wireline logging arises from the rate of drilling which is not an entirely controllable parameter. Since there is no simple way to record depth as the data are acquired, they are instead acquired in a time-driven mode. This results in an uneven sampling rate of the data when put on a depth scale. Surface software has been developed to redistribute the timesampled data into equally spaced data along the length of the well. Spring14 H. AlamiNia Well Logging Course (2nd Ed.) 28

Reservoir Rock Porosity Clay contamination (Clean or contain clay)  The presence of clays can affect log readings as well as have a very important impact on the permeability Rock consolidation (consolidated or unconsolidated)  This mechanical property will influence the acoustic measurements made and have an impact on the stability of the borehole walls as well as on the ability of the formation to produce flowing fluids. formation type (homogeneous, fractured, or layered)  The existence of fractures, natural or induced, alter the permeability significantly.  In layered rocks the individual layers can have widely varying permeabilities and thicknesses that range from a fraction of an inch to tens of feet. Identifying thin-layered rocks is a challenge. Spring14 H. AlamiNia Well Logging Course (2nd Ed.) 30

Reservoir Fluid Fluid Saturation (hydrocarbons or brine) Fluid phase (liquid or gas hydrocarbons) This can be of considerable importance not only for the ultimate production procedure but also for the interpretation of seismic measurements, since gasfilled formations often produce distinct reflections. Although the nature of the fluid is generally inferred from indirect logging measurements, there are wireline devices which are specifically designed to take samples of the formation fluids and measure the fluid pressure at interesting zones. structural shape of the rock body This will have an important impact on the estimates of reserves and the subsequent drilling for production. Spring14 H. AlamiNia Well Logging Course (2nd Ed.) 31

Well logging roles Well logging plays a central role in the successful development of a hydrocarbon reservoir. Its measurements occupy a position of central importance in the life of a well, between two milestones: the surface seismic survey, which has influenced the decision for the well location, and the production testing. The traditional role of wireline logging has been limited to participation primarily in two general domains: formation evaluation and completion evaluation. Spring14 H. AlamiNia Well Logging Course (2nd Ed.) 32

The goals of formation evaluation the presence of hydrocarbons (oil or gas)  in formations traversed by the wellbore The depth of formations which contain accumulations of hydrocarbons fractional volume available for hydrocarbon in the formation porosity Saturation (hydrocarbon fraction of the fluids) the areal extent of the bed, or geological body falls largely beyond the range of traditional well logging producible hydrocarbons determination of permeability Determination oil viscosity often loosely referred to by its weight, as in heavy or light oil Spring14 H. AlamiNia Well Logging Course (2nd Ed.) 33

Formation evaluation A number of measurement devices and interpretation techniques have been developed. They provide, principally, values of porosity and hydrocarbon saturation,  as a function of depth, using the knowledge of local geology and fluid properties that is accumulated as a reservoir is developed. Because of the wide variety of subsurface geological formations, many different logging tools are needed to give the best possible combination of measurements for the rock type anticipated. Despite the availability of this rather large number of devices, each providing complementary information, the final answers derived are mainly three:  the location of oil-bearing and gas-bearing formations,  an estimate of their producibility, and  an assessment of the quantity of hydrocarbon in place in the reservoir. Spring14 H. AlamiNia Well Logging Course (2nd Ed.) 34

completion evaluation The second domain of traditional wireline logging is completion evaluation. This area is comprised of a diverse group of measurements concerning cement quality, pipe and tubing corrosion, and pressure measurements, as well as a whole range of production logging services. Spring14 H. AlamiNia Well Logging Course (2nd Ed.) 35

Measurement types the purpose of well logging is to provide measurements which can be related to the volume fraction and type of hydrocarbon present in porous formations. Measurement techniques are used from three broad disciplines: electrical, nuclear, and acoustic. Usually a measurement is sensitive either to the properties of the rock or to the pore-filling fluid. Spring14 H. AlamiNia Well Logging Course (2nd Ed.) 37

measurement of electrical conductivity The first technique developed was a measurement of electrical conductivity. A porous formation has an electrical conductivity which depends upon the nature of the electrolyte filling the pore space.  Quite simply, the rock matrix is nonconducting, and the usual saturating fluid is a conductive brine.  Therefore, contrasts of conductivity are produced when the brine is replaced with nonconductive hydrocarbon. Electrical conductivity measurements are usually made at low frequencies.  A d.c. measurement of spontaneous potential is made to determine the conductivity of the brine. Another factor which affects the conductivity of a porous formation is its porosity.  to correctly interpret conductivity measurements as well as to establish the importance of a possible hydrocarbon show,  the porosity of the formation must be known. Spring14 H. AlamiNia Well Logging Course (2nd Ed.) 38

nuclear measurements A number of nuclear measurements are sensitive to the porosity of the formation. The first attempt at measuring formation porosity was based on the fact that interactions between high-energy neutrons and hydrogen reduce the neutron energy much more efficiently than other formation elements. a neutron-based porosity tool is sensitive to all sources of hydrogen in a formation, not just that contained in the pore spaces.  This leads to complications in the presence of clay-bearing formations,  since the hydrogen associated with the clay minerals is seen by the tool in the same way as the hydrogen in the pore space. As an alternative, gamma ray attenuation is used to determine the bulk density of the formation.  With a knowledge of the rock type, more specifically the grain density, it is simple to convert this measurement to a fluid-filled porosity value. Spring14 H. AlamiNia Well Logging Course (2nd Ed.) 39

nuclear measurements (Cont.) The capture of low-energy neutrons by elements in the formation produces gamma rays of characteristic energies.  By analyzing the energy of these gamma rays, a selective chemical analysis of the formation can be made.  This is especially useful for identifying the minerals present in the rock.  Interaction of higher energy neutrons with the formation permit a direct determination of the presence of hydrocarbons through the ratio of C to O atoms. Nuclear magnetic resonance, essentially an electrical measurement, is sensitive to the quantity and distribution of free protons in the formation.  Free protons occur uniquely in the fluids, so that their quantity provides another value for porosity.  Their distribution, in small pores or large pores, leads to the determination of an average pore size and hence, through various empirical transforms, to the prediction of permeability.  The viscosity of the fluid also affects the movement of the protons during a resonance measurement, so that the data can be interpreted to give viscosity. Spring14 H. AlamiNia Well Logging Course (2nd Ed.) 40

Acoustic measurements  formation porosity and lithology :  Acoustic measurements of compressional and shear velocity can be related to formation porosity and lithology.  formation impedance:  In reflection mode, acoustic measurements can yield images of the borehole shape and formation impedance;  integrity of casing and cement:  analysis of the casing flexural wave can be used to measure the integrity of casing and cement.  formation permeability:  Using low frequency monopole transmitters, the excitation of the Stoneley wave is one way to detect fractures or to generate a log related to formation permeability.  Techniques of analyzing shear waves and their dispersion provide important geomechanical inputs regarding the near borehole stress field. These are used in drilling programs to avoid borehole break-outs or drilling-induced fractures. Spring14 H. AlamiNia Well Logging Course (2nd Ed.) 41

well logging interpretation The one impression that should be gleaned from the above description is that logging tools measure parameters related to but not the same as those actually desired. It is for this reason that there exists a separate domain associated with well logging known as interpretation. Interpretation is the process which attempts to combine a knowledge of tool response with geology, to provide a comprehensive picture of the variation of the important petrophysical parameters with depth in a well. Spring14 H. AlamiNia Well Logging Course (2nd Ed.) 42

1. Ellis, Darwin V., and Julian M. Singer, eds. Well logging for earth scientists. Springer, 2007. Chapter 1

1. Rudimentary definitions 2. hydrocarbons presence determination 3. hydrocarbons quantity and recoverability determination 4. The Borehole Environment

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