The Slim Density Neutron Standoff Caliper (SDNSC) system in the Pathfinder LWD system provides real-time and memory recorded bulk density, photoelectric index, neutron porosity, density porosity and ultrasonic caliper measurements.
This LWD system with a nominal collar size of 43¼4-in. can perform accurate and
reliable measurements in hole sizes from 57¼8 in. to 71¼2 in. The tools density/neutron/caliper sensor packages are housed in a non-magnetic drill collar.
A separate upper LWD battery collar provides power to this tool and also puts the density and neutron measurements closer to the bit. The system acquires data then communicates to a telemetry controller via an internal bus. The acquired real-time data is sent to the surface via mud pulse telemetry. The system processes and stores all measured and calculated data for customer distribution and presentation in various log formats. It has a borehole logging capacity of over 300 hours, and can operate in flow rates up to 375 gallons per minute (GPM). It is modular and can be configured anywhere in the LWD bottomhole assembly (BHA), and has a dogleg severity of 30°/100 ft (30 m) while sliding, 15°/100 ft while rotating.
The porosity measurement systems rely on the interaction between installed radioactive sources and the rock formations, to calculate values for bulk density (RHOB), photoelectric index (PE), neutron porosity (NLIM, NSAN, NDOL) and density porosity (DPHI). All of these measurements provide the customer with vital information for accurate formation evaluation. In addition to the nuclear measurements, the system uses ultrasonic transducers to measure standoff and caliper (CALI) readings that provide information about the diameter and condition of the borehole. The dynamic standoff processing of the nuclear data eliminates any standoff effects on the density and neutron measurements.
This LWD formation evaluation tool is high pressure/high temperature (HP/HT) rated 25,000 psi, 350°F (175°C) and is an integral part of the companies Survivor HP/HT MWD/LWD services. This system is the first in the LWD industry to use a Californium (Cf252) source for neutron porosity measurements. The use of a Cf252 neutron source with reduced radioactivity and relatively short half-life is considered to be more environmentally friendly than a standard neutron Am241+Be source.

Density measurement
The density system uses a Cesium (Cs137) gamma ray source (Figure 1) with a source strength of 1.5 Curies (Ci), and a half-life of 30.2 years. The high strength steel alloy body of the mandrel around the Cs137 source has a cavity cut into it to maximize the passage of gamma rays from the source to the formations. There are two sodium iodide (NaI)/photomultiplier detector assemblies axially separated from the Cs137 source. These density detectors are encapsulated in a tungsten shielded body with windows cut into them which allow gamma rays from the formations to come in contact with these detectors, but shields any gamma rays traveling directly from the source to the detectors.
The depth of investigation for the density measurement is about 8 in. The density system primarily measures the formation densities directly in front of the detector assemblies. The detector assembly closest to the source is the near detector, while the detector assembly farthest from the source is the far detector. When the density detectors are flush against the borehole (zero standoff), the densities measured using the near and far detectors separately are the same except for small differences from the borehole fluid. When the detectors have a non-zero standoff, the two detectors will measure different densities because the volumes of mud measured between each detector and the borehole wall are different. Since the mud density is lower than the formation density, the near detector will generally read lower than the far detector. The density measurement system uses the difference between the near and far detector responses in several gamma ray energy ranges per detector and its dynamic standoff based processing algorithms to correct these borehole effects. These corrections produce a very accurate compensated density measurement.

Neutron porosity measurement
The neutron system uses a Cf252 source which has a half-life of 2.65 years. The average energy output from the Cf252 source is less than a standard LWD neutron measurement Am241+Be source, making the lower energy Cf252 neutrons more sensitive to hydrogen content (porosity) and produces a larger dynamic range in porosity response. The neutron system consists of a high strength steel alloy chassis (insert) with measurement electronics and three neutron detectors. These three He3 neutron detectors measure counts from the source and produce a ratio between the near/medium, near/far and medium/far detectors to calculate the formation porosity. The neutron count rates at these neutron detectors depend upon the matrix density of the formation. Since limestone, sandstone and dolomite have different matrix densities, the calculated neutron porosity depends upon which of these matrices is assumed. Neutron porosity can be presented as limestone (NLIM), sandstone (NSAN) or dolomite (NDOL) depending upon the matrix.
The measurements taken by this neutron porosity system are mainly influenced by the presence of hydrogen (H) in the formation and borehole surrounding the detectors. The system assumes that the H is located in the pore spaces within the formations. The neutron system is calibrated with reservoir rocks of various porosities at zero standoff. Environmental corrections are applied for the measured detector standoff, measured hole diameter, borehole salinity, formation salinity, pressure, temperature and mud weight. These corrections ensure an accurate formation porosity measurement.

Case study
In the Gulf of Mexico, Chevron required a suite of formation evaluation LWD systems to acquire real-time and recorded data in a directionally drilled 71¼2-in. borehole to a sandstone target. It should be noted that many LWD companies have had difficulties producing accurate density and neutron formation measurements in a 71¼2-in. borehole with nominal 43¼4-in. collar systems logging equipment company provided 43¼4-in. nominal collar size directional, gamma ray, resistivity, density, neutron, caliper and annular pressure services in this well. The overall objectives were to provide the customer with accurate and reliable LWD wireline replacement services. This was accomplished by acquiring, processing and presenting high resolution real-time and recorded LWD formation evaluation data (Figure 2) to define the lithology, mineralogy, formation fluid volume and types up to and including the target sandstone formation.
In the (Figure 2) LWD formation evaluation log plot, the track 1 gamma response shows a change in formation from shale to sand at x248 ft (75.6 m). The resistivity measurements in track 3 display thin interbedded formations between x248 ft to x260 ft (79.3 m). A significant increase in resistivity from x248 ft to x278 ft (84.8 m) is observed. A hydrocarbon gas cross-over response from the track 4 density and neutron measurements occurs also from x248 ft to x278 ft, with an estimated calculated porosity of over 30%. Note that the neutron and density curve scales are based on a sandstone matrix.
The LWD formation evaluation results of this well were very favorable for the customer. The formation evaluation LWD data was reliably and accurately acquired, presented and distributed. The environmentally corrected formation evaluation LWD services including the density, and neutron systems accurately defined the formations lithology, mineralogy, and fluid volume and type of the target sandstone formation.
A hydrocarbon pay-zone was discovered and characterized in a relatively clean sandstone formation. After this discovery was made, the customer continues to use the company for LWD services in boreholes of various sizes with similar results. Any questions regarding the companies LWD/MWD services can be directed to pfmarketing@pathfinderlwd.com.