The accurate calculation of porosity at the wellbore is essential for an accurate calculation of original oil in place (OOIP) or original gas in place (OGIP) throughout the reservoir. The porosity and its distribution also need to be calculated as accurately as possible because they are almost always directly used in the water saturation (Sw) and permeability calculations and, possibly, in the net pay calculations. In most OOIP and OGIP studies, only the gross-rock-volume uncertainties have a greater influence on the result than porosity does. Occasionally, where porosity estimates are difficult, porosity is the leading uncertainty. Fractured and clay-mineral-rich reservoirs remain a challenge. For this discussion, it is assumed that the core data have been properly adjusted to reservoir conditions, that the data from various logs have been reviewed and validated as needed, and that all of the required depth-alignment work has been completed.
In making the petrophysical calculations of lithology, net pay, porosity, water saturation, and permeability at the reservoir level, the development of a complete petrophysical database is the critical first step. This section describes the requirements for creating such a database before making any of these calculations. The topic is divided into four parts: inventory of existing petrophysical data; evaluation of the quality of existing data; conditioning the data for reservoir parameter calculations; and acquisition of additional petrophysical data, where needed. The overall goal of developing the petrophysical database is to use as much valid data as possible to develop the best standard from which to make the calculations of the petrophysical parameters. The second step in working with the petrophysical data is to evaluate the quality of each of these types of data. This step requires that the data inventory and database preparation steps are completed first so that this second step can occur as a systematic and complete process. The evaluation process is a "compare and contrast" exercise. The evaluation of log-data quality has many aspects. This should be noted in the petrophysical database. "Flags" of various types should be stored, for example, to denote intervals where the hole size exceeds some limit, or where there is cycle-skipping on the sonic logs. Logging tools sometimes become temporarily stuck as a log is being run. When the tool is stationary, each detector on it becomes stuck at a different depth, so the interval of "stuck" log will vary for each log curve. For example, the neutron log typically sticks over an interval approximately 10 ft above the stuck interval on a density log. It may be possible to "splice" in a replacement section of log from a repeated log section, or the invalid readings may simply be deleted. Second, each log is formally calibrated before the start of each logging run by various calibration standards. The logs are also checked again after the run. Calibration records may assist in determining the quality of the logs. Perhaps of equal importance are the written comments on the log heading made immediately after the job by the logging engineer. Third, systematic influences on the quality of log readings should be corrected. For example, if some of the wells are drilled with water-based mud (WBM), the effect of WBM-filtrate invasion on various resistivity logs can be quantified. This is done by computations made using the various resistivity logs in the same wellbore; however, where deep invasion of WBM filtrate occurs, offsetting wells drilled with oil-based mud (OBM) give a good comparison. The induction logs in OBM wells can provide accurate true reservoir resistivity values in thick hydrocarbon zones. See the chapter on resistivity and SP logging in this volume of the Handbook for more information on how invasion effects can be handled. Boreholes are not always right cylinders.
The goal of the net-pay calculations is to eliminate nonproductive rock intervals and, from these calculations at the various wellbores, provide a solid basis for a quality 3D reservoir description and quantitative hydrocarbons-in-place and flow calculations. The determination of net pay is a required input to calculate the hydrocarbon pore feet, FHCP, at a wellbore and its input to the overall reservoir original oil in place (OOIP) or original gas in place (OGIP) calculations. The total FHCP at a well is the point-by-point summation over the reservoir interval with Eq. 1. The top and base of the reservoir interval are defined by geologists on the basis of core descriptions and log characteristics. In the FHCP calculation, net pay, hni, at each data point has a value of either 1 (pay) or 0 (nonpay).
The "gold" standard for permeability is to make measurements on core samples and to determine permeability with the methods outlined in API RP 40. All other techniques are calibrated back to core measurements. However, because core measurements sample such a minute part of the reservoir, we must rely on techniques that can be applied in a widespread fashion across the reservoir. These methods rely on measurements on sidewall samples, correlation to wireline logging responses, interpretation of nuclear magnetic resonance (NMR) logs, wireline formation tester pressure responses, and drillstem tests. This technique is valid for slightly to unconsolidated sandstone rock types.
The Bowland Shale is a Carboniferous formation of Asbian to Yeadonian age located in Northern England. It is estimated to have a shale gas section with more than 5,000 ft thickness holding over 1300 TCF of total original gas in place. Drilling in the Bowland Basin started in August 2010 and by the end of 2011, three vertical wells (PH-1, GH-1 and BS-1) were drilled to a depth of 8,860 to 10,500 ft with objective of logging and coring the potential shale gas formations including Upper Bowland, Lower Bowland, Hodder Mudstone and Sabden Shale. All the drilled wells encountered several borehole stability problems, such as tight-hole, pack-off, overpull and excessive cutting, causing significant non-productive time (NPT) during drilling. Specifically, in GH-1 and BS-1, side-tracking was required to reach the target depth which imposed significant cost to the project. Careful investigation of the recorded drilling problems showed that majority of them were associated with formation collapse due to insufficient drilling fluid pressure. Fluid losses also occurred in some of the formations due to either too high of downhole pressure or presence of critically stressed natural fractures. These incidents implied that the applied casing design and mud weight program were not appropriate for the current-day state of stress and rock properties.
A comprehensive experimental and analytical geomechanical study was carried out to develop a reliable borehole stability model that can firstly explain the observed drilling incidents and secondly provide guidance for design and drilling of the planned wells. The plan was to drill a S-shape appraisal well (PNR-1) in the Preston New Road exploration site to log and core the Bowland Shale sequence and select the optimum landing depths for subsequent horizontal sections (PNR-1z and PNR2) to be completed for multi-stage hydraulic fracturing. The study recognized intrinsic shale anisotropy as a primary causative factor for borehole stability issues and formation collapses in the offset wells. Other important factors were identified to be the abnormal pore pressure regime and the presence of a tectonic strike-slip stress regime with large horizontal stress anisotropy. The anisotropy of the Bowland Shale was characterized in both laboratory and field scales, and anisotropic wellbore stability models were developed for offset and planned wells. As a result of this study, the PNR-1, PNR-1z and PNR2 wells were successfully drilled and completed with no notable borehole stability problems despite the presence of narrow operating mud weight window in several stratigraphic intervals. This paper presents a summary of the conducted borehole stability analysis aiming at risk-free and successful drilling of horizontal wells in the Preston New Road exploration site with emphasis on the effect of shale anisotropy.
Carr, Timothy (West Virginia University) | Ghahfarokhi, Payam Kavousi (West Virginia University) | Carney, BJ (Northeast Natural Energy, LLC) | Hewit, Jay (Northeast Natural Energy, LLC) | Vagnetti, Robert (National Energy Technology Laboratory, US Department of Energy)
Distributed temperature sensing (DTS) was used to record temperature from early 2016 to present for a Marcellus Shale horizontal dry gas well, MIP-3H, located in Monongalia County, West Virginia. In addition, after wellbore clean-out with water and nitrogen a flow scanner production log was conveyed on March 02, 2017. The flow scanner provides one day of gas and water production from each of the 28 stages in MIP-3H and from each of the clusters. The DTS data provides an opportunity to inspect the reservoir for Joule-Thompson (JT) effect, a phenomenon that describes cooling of an non-ideal gas as it expands from high pressure to low pressure, and obtain a relative production attribute along the lateral of the MIP-3H. The original fiber-optic DTS data shows the temperature along the lateral; however, due to the geometry of the well with toe up and the presence of a small fault and minor water production at Stage 10 relative gas production of each stage cannot be directly determined from the raw DTS data. We present two methods to generate DTS attributes that can be used to better reveal relative gas and water production through time from each perforation cluster and each stage of the MIP-3H. The first attribute deals with the deviations of the DTS measurements from the calculated geothermal temperature, while the second attribute calculated the difference between DTS temperature and the average daily DTS temperature along the lateral of the MIP-3H. We show that the latter DTS attribute provides a more robust image of temperature variations regime along the lateral than the former attribute. Negative values of the DTS attributes reveals JT cooling, resulting from stages of the MIP-3H with higher natural gas production. A correlation analysis of the production log with the calculated DTS attributes suggests that the production log is not representative of the entire production life of MIP-3H well. Temporal correlation with the DTS attributes is highest close to the production log recording day (March 2, 2017) decrease rapidly and the weak correlation switches from positive to negative.
Electric Submersible Pumps (ESP) are commonly used artificial lift equipment in production wells. The ESP packer penetrator system is designed to carry the electric power cable that connects the electric motor in ESP to the surface control panel. Various chemicals downhole make up highly corrosive and hostile environments to the metal wires and their insulation materials for electric connectors. Many ESP failures could be attributed to packer penetrator failure due to corrosion of the electric connector beneath the ESP packer.
A method is developed to generate a low density gel system that isolates the electric connector from downhole chemicals in order to provide prolonged protections of electric connectors against corrosive atmospheres and chemical attacks. Mixture of low-density materials/composites are prepared on the surface and then pumped into targeted place through the bypass tubing. The mixture has low density so that it travels upwards in the wellbore and floats on the top of downhole fluids. Under a given well temperature, a rigid gel/composite system forms between the electric connector and the downhole fluids, isolating the electric connector from the hostile chemicals thus providing a better protection. We have developed the low density settable material and demonstrated its performance in the lab scale. It is also scaled up to a mocked up physical simulator to observe the flow dynamics and chemical reaction in realistic geometry.
Electric Submersible Pump (ESP) is an important artificial lift technology for boosting well production (Tacks 2009). Its main advantage over other artificial lift methods include high rate and the ability to produce wells to abonnement. An ESP system mainly consists of a centrifugal pump, a protector, power delivery cable, and a motor. The pump is used to lift well fluids to the surface. The motor provides the mechanical power required to drive the pump via the shaft. The power delivery cable provides a means of supplying the motor with the needed electrical power from the surface. The protector absorbs the thrust load from the pump, transmits power from the motor to the pump, equalizes pressure, provides/receives additional motor oil as temperature changes and prevents well-fluid from entering the motor. The pump consists of stages, which are made up of impellers and diffusers. The impeller, which is rotating, adds energy to the fluid to provide head, whereas the diffuser, which is stationary, converts the fluid kinetic energy from the impeller into head. The pump stages are typically stacked in series to form a multi-stage system that is contained within a pump housing. The sum of head generated by each individual stage is summative; hence, the total head developed by the multi-stage system increases linearly from the first to the last stage.
A miscible injectant was used in a single injection well pilot in the Yates field to mobilize remaining oil in the gas cap and accelerate gravity drainage. Nitrogen, CO2 and recycled gas injection, all immiscible with Yates oil due to low original and current reservoir pressure, have been used historically to assist the gas-oil gravity drainage (GOGD) development. The result of immiscible injection has been a lowering of the gas-oil contact, a thinning of the oil column, and leaving a remaining oil saturation in the gas cap of up to 40 percent. A hydrocarbon mixture rich in ethane and propane and miscible with Yates oil was injected in a CO2 injector for six months after which the well was returned to pure CO2 injection.
Data collection during the pilot included repeat saturation logging of a newly drilled observation well, well tests of nearby horizontal producers, frequent gas and oil sampling, and chromatographic analysis. Phase behavior PVT experiments were also conducted which confirmed miscibility of the injectant and improvement over CO2 injection. Numerical simulation of pilot performance was also used as part of the interpretation.
Pilot results to date show a doubling of oil rate at peak over base oil decline, breakthrough in horizontal producers within 3-5 months matching an a priori prediction from numerical simulation, 10 percent reduction in oil saturation in the target interval in the gas cap, and the return of two wells to continuous production after having been shut-in due to high gas-oil ratios. Early interpretation of pilot results showed that most of the incremental oil and back produced enriched hydrocarbons came from one well. During the follow-up CO2 injection phase, one of the horizontal wells completed in the gas cap (unlike other pilot producers), was redrilled deeper into the oil column to improve the pilot areal and vertical sweep.
The pilot design, results, and interpretation will be discussed. Results from the pilot will be used to support evaluation of a field wide development, which could lead to substantial incremental reserves and extension of the field life.
Douglas is an oil field in the Irish Sea where electric submersible pumps (ESPs) are used as artificial lift method. This paper presents the ESP failures and root causes in early stages of production, and the improvements implemented that contributed to the outstanding improvement in ESP run life. These improvements increased field production, minimized deferred production, and minimized workover costs.
Every ESP system pulled from the Douglas field was dismantled, and a failure analysis was performed to identify the failure root cause. When the root cause is confirmed, applicable equipment upgrades were recommended to address the failure modes and increase ESP run life. In order to monitor system performance, reliability was tracked and compared over the years. The ESP reliability was analyzed using data gathered from ESP run life and failure analysis covering more than 20 years of ESP operation in the Douglas field.
Failure analysis performed in the pulled strings indicated failures related to poor electrical power quality. A power study was performed on the Douglas platform, and it was observed that the wells exhibited natural system resonance frequency amplifying drive harmonics. Resonant peak frequencies were observed between five and seven kilohertz, and voltage impulses were observed up to nine kilovolts. Such impulses were weakening insulation in some downhole components, such as penetrators, cables, and motors. Following the recommendations from the study, load filters were installed in every well to eliminate voltage overshoots. The filters provided cleaner voltage and current waveforms to the system, power cable, and at the motor terminals.
In addition to the power supply issues, it was noted that there was H2S contamination in the protectors and motors pulled from the field. In order to increase ESP reliability, an advanced H2S scavenger protector design was introduced. The advanced protector has features that delay the ingression of H2S into the motor and copper liners that serve as sacrificial parts to be consumed by H2S.
The solutions proposed for ESP operation in combination with ESP design improvements helped to improve ESP reliability considerably allowing ESP systems to achieve outstanding run lives in the Douglas field.