The following case study was taken from “High-Temperature LWD (Logging-While-Drilling) Suite Provides High-Quality Data for Correlation Purposes in Complex Structural Area: A Case History from Po Valley Deep Triassic Region,” by S Mazzoni, M Borghi, ENI Div. E&P; G Affleck, R Christie, M Troiano, Weatherford International, presented at the Offshore Mediterranean Conference and Exhibition in Ravenna, Italy, 25-27 March 2009.
In the deep, high-pressure, high-temperature wells in the Villafortuna/Trecate field in northern Italy, LWD was planned to enable continuous correlation for casing and coring points. Of two required targets for the well, the first was not assured because of possible faults. The second showed risks for depleted intervals, with very poor seismic control.
The field consists of two carbonate oil reservoirs with the presence of intense tectonic phenomena: the upper reservoir (Conchodon Dolomite and Dolomia Principale) and the lower reservoir (M.S. Giorgio Dolomite), overlayed by a thick carbonatic and terrigenous sealing sequence. Analysis showed the possibility of draining an oil mineralized area in the lower reservoir.
A sidetrack was planned from a previous well. Well profile included entering vertical at the top of the lower reservoir, but the sidetrack also had to verify the presence of mineralisation in the upper reservoir. The well plan called for a 8 ½-in. sidetrack from the 9 5/8-in. casing to the Medolo formation upper interval, followed by drilling a 5 ¾-in. hole to target.
Due to uncertainties about the formation type at target and the formation mineralization, cores were to be collected from the lower reservoir.
Because standard LWD equipment had experienced numerous failures at downhole circulating temperatures up to 160°C in the previous hole section, it was decided to use an HPHT LWD system to drill the 5 ¾-in. sidetrack. Weatherford provided a 4 ¾-in. suite of HPHT LWD tools, including directional MWD, pressure-while-drilling (bore and annular), multi-frequency resistivity and azimuthal gamma ray sensors.
To drill the sidetrack, each collar was powered by a dual-battery module assembly consisting of two high-temperature intelligent batteries that enabled logging at temperatures up to 200°C.
These lithium batteries had two important operational considerations. One is the maximum safety factor before the cells become unstable and vent. Internal testing has shown that the cells will vent once the internal temperatures reaches 212°C.
The second consideration concerns operation of the equipment at temperatures below 70°C, such as during deck tests and shallow hole tests. Extended use at low temperatures can cause high and erratic current usage, which can shorten the life of the batteries. The battery thermal protection function meant the pulser will not work until the temperature of 70°C is seen, allowing for extended life.
The MWD system was designed for hostile-environment logging. It provides real-time directional and logging data through a pressure-modulated telemetry system for HPHT drilling environments. It is able to operate at temperatures up to 180°C, survive at 200°C and withstand downhole pressures of 30,000 psi. An integrated directional sonde uses three orthogonal high-temperature accelerometers and magnetometers housed in a nonmagnetic titanium alloy chassis to provide directional and toolface measurements that track the drilling path and orient the PDM motor during sliding operation.
The suite was also equipped with a pressure-while-drilling sensor and programmed to store all pressure data in memory every five seconds and all temperatures every five minutes. The result was continuous, highly accurate downhole measurement while drilling, wiping or tripping in/out of the hole. Bore pressure, annular pressure and temperature, and ECD were transmitted and plotted real-time to help minimize mud losses and optimize the mud program.
To log the well, a high-temperature multi-frequency resistivity tool was combined with a high-temperature azimuthal gamma ray sensor, both rated to 180°C operating and 200°C survival temperatures and 30,000 psi operating pressure. All data were transmitted to surface by the driver and pulser in the MWD system.
The high-temperature multi-frequency resistivity sensor used three independent transmitter-receiver antenna spacings and two electromagnetic wave frequencies to provide highly accurate measurements in the extreme conditions, with 12 fully compensated phase and attenuation measurements recorded every 10 seconds.
The real-time-transmitted 2 MHz shallow, medium and deep-phase resistivity curves were useful in formation identification and continuous correlation for coring and casing points.
Gamma ray information was acquired using a high-temperature azimuthal gamma ray sensor consisting of five banks, each with two Geiger Muller tubes. The number, size and symmetric distribution of tubes were chosen to provide the optimal combination of statistical precision and azimuthal sensitivity and to allow azimuthal measurements while rotating and/or sliding.
Triggered azimuthal gamma ray data in quadrant format was also transmitted in real time to maintain a good data density. The combination of total gamma ray, azimuthal and resistivity data while drilling made formation identification and continuous correlation for casing and coring points possible.
Gamma ray data in octant format (8-sector borehole), sampled and stored in memory every 10 seconds, was sent to the office at the end of each run to be analyzed and processed with specialized software.
Image logs and dips interpretation played an important role in improving the characterization of reservoirs by showing graphic details about texture and structural features, such as faults and fractures, and by identifying dips and defining the angle and direction of tilt in sedimentary layers.
In addition to the temperature and pressure measurements recorded real time, 12 resistivity measurements and oriented azimuthal gamma ray measurements were acquired while drilling.
The azimuthal gamma ray sensor records eight values around the borehole in memory, and sends four values up hole in real time. These values are oriented using the directional package, then processed and mapped to a false colour palette to provide an image of the borehole. The resulting imaging log is depicted in a reversible dark/light scale that portrays high/low gamma values.
Two different images are typically generated: a static image, where the minimum and maximum values of the measurements for the entire data set are mapped to the colour palette; and a dynamic image, where a user-defined depth range determines the minimum and maximum values, which are then mapped to the colour palette. The static image is an absolute scaling of the measurements, whereas the dynamic image is a relative one, which also provides better contrast in areas where there are small changes in absolute values.
After a correctly oriented image is made, the bedding planes can be delineated. Planes in 3D space are represented in 2D using sinusoids. The geoscientist picks the sinusoids which represent the apparent dip angle and azimuth of the bedding plane. Using the directional information, true dip is calculated from the apparent dip and can be referenced to either magnetic north or true north by correcting for magnetic declination. The dips are then plotted using tadpoles.
The main objective was to use the gamma ray and resistivity data to correlate the sidetrack well with the offset well, in real time while drilling, to set casing shoe between overpressure zone and depleted zone, to identify coring point and identify formational variations. This was achieved despite an in-situ static temperature of 180.5°C and a circulating temperature of 160°C.
Several benefits were identified to logging high-temperature gamma ray and resistivity data in real time:
1. The real-time continuous gamma ray curve and resistivity curve permitted the operator to correlate while drilling the sequence crossed in the sidetrack well with the similar sequence crossed in offset wells.
2. The real-time correlation made it possible to identify the presence of a main direct fault and to estimate its reject. The upper reservoir was identified 136 m higher than the prognosis estimation.
3. The continuous real-time gamma ray and resistivity curves allowed the operator to better define the stratigraphic setting. This definition was critical because the use of a PDC bit often destroyed the lithological characteristic of the drill cuttings.
4. The continuous real-time gamma ray and resistivity curves allowed the operator to identify a carbonate sequence in the lower section, which prompted the decision to cut a core at bottom. The core results, which showed the rock to be very tight, and log correlation resulted in the decision to plug and abandon the lower section and to produce the upper reservoir with very good results. New wells in the project are planned as a result of this well.
5. Real-time annular borehole temperature gave the operator an idea of the temperatures present in the sequence.