- Logging While Drilling
- Permeability Measurement
- Reservoir modelling
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While mudlogging is essentially contracted out to specialized engineering firms, the wellsite geologist working on the wellsite needs to understand this side of the business. among the duties of the mudlogging unit are:
Monitoring drilling parameters and process
Mudloggers monitor the drilling activity and send in a Daility drilling activity report to the WSG. They track every activity going on at the rig floor maintain a daily drilling activity log. With these, any experienced oil field personnel can retrace exactly what went on all day on the rig. The wellsite geologist reviews this document and sends it to the operations department in town.
The drilling parameters, most of which are recorded by the various sensors, are:
Other drilling parameters
These help the Mudlogging Unit to monitor and report drilling process. The Mudlogging team will provide the WSG a time and depth ASCII files each day showing these drilling parameters variation with time and with depth respectively. They will also provide:
b. Cutting sample collection and description
Most of the cutting samples are collected from the shakers. Samples are collected every 10m, 5m or as may be requested by the drilling program or final well report. If the ROP is very high it will become impossible to safely catch samples in time. This is becomes over the required distance is already drilled before the sample catcher could go to the shakers, prepare and bag the sample, then come back again. If this is the case the sampling interval should be increased. “Double bagging” of samples (where sample from same unit is bagged twice to meet up with uncollected sample) is a waste of time as it produces invalid data.
Depth is the first important factor to consider during any logging process; it is one parameter that both MWD , WIRELINE and LWD shares. However the depth of interest is not only the depth at which the tool is evaluating the formation (depth of measurement), but into the formation it is evaluating. The later is known as depth of investigation, which is a measure into the formation. Often the depth of investigation depends on the strength of the signal, the parameter measured or emitted and the tool spacing or acquisition condition. This is especially important in LWD since the rate of penetration (ROP) of the drilling bit and tool may be too fast for a proper measurement of depth investigation. ROP may be adapted for proper logging, as well as drilling mud flow rate.
One way of assessing the effectiveness of the intended result of LWD is the resolution, the depth of investigation and the log quality control after logging.
Physical resolution is the smallest increment which a tool can measure and this defines the separability and recoverability of the data.
Axial resolution is the measurement per rotation and this could give the number of points assessed depending on chosen speed of rotation and descent of the tool.
Tool speed is only one of the things which could be controlled in other to obtain good logs. Others are the rate of penetration (ROP) of the tool. Despite a carful monitoring of logging procedures, intrinsic or external situation (such as borehole conditions) could still result in problems with logs.
Some of the problems that could result when logging are:
Again, proper tracking of the depth at which the following tools measure formation properties is important. Usually a depth encoder is placed on the draworks to measure depth during drilling. LWD comes with its depth sensor. An example is the Surface Sensor for Depth (SSD) provided by Schlumberger.
Looging while drilling can, among others, measure:
Intrinsic permeability is an important factor for evaluating the hydraulic property or fluid flow in porous media. Its measurement in the laboratory at different confining pressure is one way to predict the flow of hydrocarbon in real geological systems. Ideally, intrinsic permeability is a measure of the mobility of fluid within a porous material and as such related only to the pore spaces or geometry of the rock. This is therefore independent of the fluid type.
In real cases, the type of fluid used and the laboratory equipment could induce relatively different results for the same sample, although these results would be close to the actual value known as the absolute permeability. The difference can be corrected, particularly in very low permeability measurement where the tendency of deviation from the real value is optimal. In this work, the gas permeability was checked for consistency and corrected to obtain the absolute permeability. Systematic errors were also checked and random errors estimated to ensure a certain degree of accuracy before interpreting the results.
Gas permeability and correction to obtain liquid permeability is preferred to direct measurement for water permeability because
(1) there is Bingham flow when pore spaces are small and water molecules adhere to pore wall causing resistance to flow
(2) N2 gas is chemically inert and does not react with the sample or tubes,
(3) the compressibility and viscosity of gas are less sensitive to temperature changes, and
(4) there is availability of accurate commercial gas flow-meters.
Standard flow meters were used for the permeability measurement in this study.
However, some of the samples from the fault gouge, being of very low permeability, could not show any flow in the standard flow meters. A “bubble displacement” method was adapted for such samples, where the gas from the confining chamber is channelled to a 1mm-diameter tube with some water drop. This water drop that forms a bubble-type layer was displaced upwards on the tube and the amount of displacement over a given time (read-off from a stop watch) used to calculate the rate of flow.
Reservoir evaluation in an oil field
At the end, the aim is to evaluate the reservoirs, target and drill through them to produce the hydrocarbon resources. Reservoir evaluation aims at describing the characteristics of a reservoir such as porosity and permeability, their size, shape and general geometry. Different types of reservoir show different characteristic. Some of these characteristic thick are intrinsic, meaning that they are inherent in the reservoir. Other relate to how they will behave respect to flow or how they relate to each other, which are described below.
The main reservoir properties: this will include properties inherent to the reservoir, such as porosity.
Reservoir lithology: lithological description will highlight the various lithological facies.
Size and structure: this will include such important factors as the sizie and geometry of the reservoir bodies vertically and laterally.
Reservoir Quality: the reservoir quality groups several factors, including fracturation for example, degree of cleanliness and to some degree its flow properties such as permeability.
Clean and dirty reservoir: this is the evaluation of the degree of shaliness of reservoir (presence of clay means dirty while absence means clean) and the presence of cements and other signs of lithification.
Useful tools and methods of reservoir evaluation
Reservoir modelling at block level
The purpose of reservoir modelling at block level is to puts in a representative and functional scale (often visual), the relationship between most if not all of the reservoir parameters evaluated. To carry out reservoir simulation, divide the field into small cells with properties relatively easier to qualify and quantify; it is over these small discrete units that parameters such as permeability, pressure, mobility, and these is made to run across the cells to simulate how they will behave during production.
Reservoir geologists and engineers make assumptions about possible mixing of the oil and gas fluids, testing which values and flow patterns will give a coherent result. Static model concerns aspects relating to the geological conditions such as geometry, shape, size, while dynamic models relate to flow and other behaviours resulting due to changes in pressure condition.
Wireline is the more traditional logging approach. It was first used in Northern France, where eletrical resistivity was measured on rock formations. Wireline logging tools are also known as Openhole logging, because the hole is drilling and the drilling string pulled out before it is carried out. Openhole logging can either be Pipe conveyed, Coiled tubing conveyed or Tractor conveyed. Some data measured by Wireline are:
Depth is the most important parameter logged; other parameters are tied to it and cannot be properly interpreted if their depth is not known. There are two types of depth: driller and logger depths. The driller’s depth is read out using the depths of the drilling pipes whereas the logger’s depth is based on the logging tools or cable decended into the hole. Modern rigs have a sensor known as Depth Encoder, which regords this depth changes in real-time. Because a well is most stable during (wireline) logging, the logger depth is considered more precise. The logger depth is corrected for errors such as wheel error (wear and slippage) and for stretch; a chart is used to correct the effect of stretching on logging cables. The final depth is tied to the the gamma ray log, which is used to correlate the other logs.
Most logging tools, especially lotho-density tools, have a caliper that measures the size of the hole. The size of the hole can indicate lithology as different lithologies as difference lithologies will packoff and form cavings or mudcake (mud filtrate on well walls) in different ways. Some calipers serve to stabilize well tools, by placking at the walls and centralizing the tool. Some have been adapted to take migro-images of the well.
The nuclear method exploits the radioactive elements in nature such as Potassium, Thorium and Uranium. The particles emitted by these elements are measeared in nuclear logs. Radioactive elements are found in organic matter (uranium for example) and reducing environment (such as bitumen and shaly clays). Thorium is found in heavy mineral while potassium rich clays and sometimes found in sandstones.
The three main nuclear methods are:
Gamma ray is the main lithological log, especially in sand-shale sequence of rock units. Gamma ray measurement is used to estimate the Vshale of a rock.
Vshale from Gamma Ray, at any point of Measured GR,
VshGR = (GRmeassured – GRmin)/ (GRmax – GRmin)
Vshale or Volume of shale is used in evaluating how clean a reservoir is. It can be calibrated locally, such that a certain percentage of it, say 70% represent the VClay in that lithology. VClay is used for estimating the sealing potential of caprocks and fault zones.
Neutron measures the emitted neutron (of hydrogen atoms) by radioactivity of the formation. It is often interpreted in combination with density. The density log measures an apparent density of the formation known as RhoB or Bulk Density, since it measures also the density of the overall package in the rock, including the fluids.
The Density, Neutron and Sonic/Acoustic are known as lithology-porosity logs.
An acoustic or sonic log uses the speed (or slowness) of sound to give the property of a formation. Slowness is the inverse of speed in logging terms. The principles of the acoustic log are that waves will move with different speed in different rock types, and depending on the pore spaces, density and other properties of the rock.
An important characteristic of acoustic tools is that their positioning inhole; it has to be centralized, especially in large holes. This is important because it is the sound, relative to distance to formation, which is measured. Presence of fractures and dipping bed will lead to disturbances. A bad positioning will lead to what is known as spiking.
Spiking result from poor tool positioning due to:
In resistitivy logs, the resistance (or conductivity) of a rock unit is measured, by induction or by a laterolog measurement process. The conductivity is the inverse of resistance, and it is the ability of a material to allow current to flow in it.
Cased-hole Cement Evaluation
Cement evaluation logs are often cased-hole measurement, wireline measurements, carried after a cementing operation and before drilling the next section of a well. There are two major uses, to verify: Casing to cement bond and Cement to formation bond.
-After obtaining either wireline or LWD data, its usefulness depends on the ability to interprete it and optimise subsurface information from it. Whether wireline or LWD, the basic principles of interpretation is the same. But there are differences in quality of data and the necessary QC before interpretation. Wireline gets one final data once acquired, but LWD will have a RT and a Memory Data. The memory data is more reliable and is used once the actual drilling is over, whereas the RT helps with immediate data interpretation while drilling.
Log interpretation involves extrapolating measured petrophysical and other downhole data, deriving other data, in such as way that that geological information are obtained, thereby leading to a better understanding of the objective in a subsurface formation. Here are further definitions to serve as reminder for explaining some aspects of log interpretation, most of the above has been mentioned in previous sections.
Porosity: the amount of pore space in a unit volume of a rock or formation, which is the total volume that will be occupied by fluids if present
Permeability: the ability of a fluid to flow through a rock unit which depends on the connected pore spaces and on other fissures.
Saturation: the fraction of pore space occupied by water, oil or gas, which gives the water oil or gas saturations respectively.
Resistivity: the resistance offered by a given rock unit to current, is is the opposite of the conductivity of that rock.
Spontaneous Potential (SP): the electrical potential given when formation connate (i.e) water is made to interact with water based mud (which is a conductive drilling fluid) in the presence of some (shaly) rock with free ions. When combined with gamma ray log it can find usefulness in delimitating lithological boundaries and in lithology identification. It can also give an estimation of porosity using specific charts.
Gammer ray: a measure of the natural resistivity in a rock formation, which can be indicative of lithology in a sand shale sequence.
Vclay and Vshale: the amount of clay mineral in a rock unit. This is different from the Vshale, which is the amount of shale in a rock unit. Vclay can be estimated from Vshale if the amount of clay per given volume of shale has already be established of a given region.
Bulk density: the density of a rock unit and the fluid it contains
Vertical seismic profiling: this is the acquisition of seismic from a well; usually the geophone and receiver are placed in hole and on surface. This can help refine existing seismic while (seismic while drilling – SWD) or after drilling (4-D seismic).
Coring refers to the process of cutting through a rock body in a cylindrical manner in order to extract a sample of the rock from the formation. A core is the sample of rock cut, which is removed from the wellbore during the drilling programme and after the core phase has been drilled through. It is probably the only physical representation of the formation, albeit in limited size.
A core provides:
A core can either be vertical, horizontal or lateral (side wall core). The wireline rotary method can catch several (up to 40 or more) sidewall samples in one run, but isb limited in that it is taken from the sidewall which may be invaded or flushed – also often damaged.
Aim of witnessing a core job is to ensure a safe, best quality log and maximize recovery and efficiency of the coring process. These is assessed by:
Recovery = core cut per recovered at surface
Efficiency = cut per barrel length
Usability = amount of core that can be (plugged) used for core analyses
Among the coring technology providers in the industry include:
Most of the core companies offer similar technologies, with slight variation. In general core service merit there relatively low additional cost when job is well done. The aim is to obtain a good quality core that maximizes the amount recovered to the length of barrel run.
Core data interpretation
Uses and application of core data is a very wide area, and what you can do with it depends on zhat you want and the need to invest and use the right methods. Data interpretation could be done inhouse in the company or subs to a consultancy or the core company.
Just to name a few major areas, cores can be used to evaluate:
Structural trends, fractures and tectonics
Reservoir properties, compartmentalization and nature of fluids
For seal evaluation, shaliness and sandiness of units, Shale Gauge Ratio determination
Other Geochemical and petrophyhysical evaluation
For the oil company, the ultimate aim will be to determine the OIP, the core gives them the real data and real lithology.