NMR logging is primarily used to determine porosity independent of lithology and permeability. It might also be used to address some unsolved log interpretation problems with conventional Resistivity-Nuclear logs. We investigate three separate applications:Hydrocarbon identification in low contrast resistivity pay zones. Central Arabia sandstones reservoirs are often difficult to interpret because of unknown water salinities. There is little resistivity contrast between water-bearing reservoirs and oil-bearing reservoirs. The water-bearing reservoirs contain relatively fresh water, and thus show relatively high resistivity readings. The resistivity in the oil-bearing reservoirs is variable because the reservoirs contain fresh or salty water. Often, the oil-bearing reservoirs show a high level of irreducible water saturation that depresses further the resistivity reading, thus making the pay identification from the resistivity log extremely difficult.We used NMR logging with a modified Differential Spectrum Method (DSM) to isolate the hydrocarbon signal from the water signal. The modified DSM has a third pass at very short Wait Time (WT) which allows the computation of three DSM spectra instead of one from conventional two pass DSM. Pay zones are those that show mostly the hydrocarbon signal after removing the water component from the total signal. The technique works well in this so-called "low-contrast resistivity", but "high-contrast NMR relaxation" environment.Capillary pressure and permeability determination. Capillary pressure depends on pore throat size which is often a function of pore size in clastic-like sediments. NMR log gives pore size distribution that might be used to derive capillary pressure curves. Capillary pressure curves are the key to understand NMR permeability. We show that NMR permeability is lithology dependent, and that T2 cutoff can be derived from NMR log and conventional core analysis of permeability. T2 cutoff is essential to determine bound fluid (BF) and free fluid (FF) volumes to predict water production. No other conventional log gives this information.Residual Oil Saturation (ROS) determination. ROS determination using NMR logging technique has been performed in a carbonate reservoir. We used a two-pass technique: a first pass to determine the total fluid volume (water+oil), followed by a second pass to determine the oil volume after doping the mud with Manganese Chloride to kill the water signal. ROS is calculated as the ratio of the oil volume over the total fluid volume. Introduction NMR logging technique is well publicized and was introduced in the kingdom of Saudi Arabia in May 1995. Shortly thereafter, Saudi Aramco started the evaluation of NMR logging because of its potential to resolve many unanswered log interpretation problems. We have reviewed the published papers on direct hydrocarbon typing, permeability relationship with capillary pressure data, and residual oil saturation (ROS) determination using NMR logging in view of applying the techniques in the kingdom. The above applications are important and are unique to NMR logging. In retrospective, we have improved the techniques and applied them to our reservoirs which comprise fresh-water shaly sandstones as well as complex carbonates. This paper summarizes our findings and contribution to the field of NMR applications from the viewpoint of the reservoir engineer. Hydrocarbon Detection in Low-Contrast Resistivity Pays Central Arabia sandstones reservoirs are often difficult to interpret because of unknown water salinities. There is little resistivity contrast between the fresh water-bearing reservoirs and the oil-bearing reservoirs. We used NMR logging with a modified Differential Spectrum Method (DSM) to isolate the hydrocarbon signal from the water signal. P. 127^
TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractState-of-the-art Nuclear Magnetic Resonance (NMR) applications can be classified into two categories: (1) basic applications designed for total porosity, bound water and permeability, (2) advanced applications that address hydrocarbon type, viscosity and flushed zone hydrocarbon saturation, amongst some others.An NMR log designed for basic applications -especially a T 1 log -can provide accurate porosity information in shaly sands and complex carbonates; answer questions related to reservoir quality and producibility; and easily identify gas or tar zones. Given the obvious benefits of such information for routine petrophysical analysis and timely business decisions, one would assume that NMR logging would already be an everyday phenomenon in formation evaluation. Unfortunately, NMR has yet to enter the main stream of everyday logging: it is still considered a specialty, requiring the involvement of experts from the planning to the interpretation stages.Partially due to historical reasons, but more so for cost justification and product differentiation, service companies have traditionally focused on advanced applications. Hence, despite significant advances made in all phases of the technology, concentrating on sophisticated methodologies has resulted in slow logging speeds and high costs, in addition to complex data acquisition, processing and interpretation procedures.Turning the everyday-NMR concept into a reality will require standardization of the service and proper hardware that is free of the limitations of the existing tools. The main objective of this paper is to discuss specific strategies to achieve the everyday-NMR goal for wireline and LWD logging. The proposed strategies include not only the technical requirements, but also the initiatives required from both the operators and service companies.
Calcium carbonate scale represents a significant safety and operational problem in offshore carbonate fields operated by Saudi Aramco. Scale can form on any surface where the pressure drop is sufficient for the produced fluids to form the scale and unload entrained solids. These solids will deposit on wellhead equipment such as the wellhead surface valves, piping, well tubulars and may plug the perforations. Well safety is jeopardized by hindering the operation of critical safety valves such as subsurface safety valves (SSSVs) and surface safety valves (SSVs). Further, scale buildup can cause backpressure problems in the tubing as the pipe internal diameter is reduced. Identification of scale in impacted wells requires periodic inspection and repair. This paper presents Saudi Aramco's experience in eliminating calcium carbonate scale by treating existing scale using HCl acid. Scale mitigation was initially attempted using an encapsulated inhibitor placed in the rat hole of vertical wells. This method had a limited treatment effective life and Saudi Aramco has moved to using inhibitor squeeze treatments. Chemical squeezes places inhibitor (phosphonate-based) directly into the formation of wet oil producers and is now the currently employed method of long-term prevention of scale. Case studies using both methods of scale mitigation are discussed as well as future plans to improve the effectiveness of these treatments. Introduction Field "B" is located both onshore and offshore along the western edge of the Arabian Gulf. The field is a north to south trending, elongated, anticlinal structure which measures 40 Km in length and 19 Km in width. This field was discovered in June 1964 and oil production began in 1967. Field "B" is a multi-reservoir field with 11 oil-bearing reservoirs at various depths from 2,133 to 3,048 meters (7,000 to 10,000 ft). The crude grade is Arab Extra Light crude with 7.5 mol% H2S and 4.5 mol% CO2. The two main reservoirs in this field are the HN and HD. The main productive formations have a low permeability in the range of 1–50 mD. The HN and HD reservoirs are carbonates reservoirs. Bulk XRD analysis of the HN reservoir cores indicates that the zone of interest contains 97–100 wt% calcite and 0–3 wt% ankerite. On the other hand, HD reservoir cores contain 70–92 wt% calcite, 0–30 wt% dolomite and 0–5 wt% ankerite.1 For pressure support purposes, peripheral water injection began in 1973 using 14 injectors into the HN reservoir and 28 injectors into the HD reservoir. The injection water is drawn from a shallow aquifer. The produced water is injected into a separate, segregated disposal system. Water breakthrough first occurred in the HN reservoir in mid 1975 and in late 1978 in the HD reservoir. Water-handling facilities were placed in service in 1983. The water cut for both the HN and HD reservoirs has gradually increased since the commissioning of these facilities and currently the field produces crude oil with an average water-cut of 32 vol%. The first scale build-up problem was encountered in 1987. Since then, scale became a difficult problem to manage as more wells started to produce formation or injection water. Scale build-up has resulted in several operational problems and production losses. The main objectives of this study are to:give a brief summary on the scale problems encountered in Field "B" carbonate reservoirs,discuss how this problem was addressed, andsummarize field experience gained from solving this problem. Chemical Analysis of the Formation Brines The composition of the produced water varies significantly for the HN and HD reservoirs as noted in Table 1. The Total Dissolved Solids (TDS) for the HN reservoir brines varies from 27,000 to 230,000 mg/L. The calcium ion concentration is in the range of 1,904–18,876 mg/L. The TDS for the HD reservoir brines is higher than that of the HN brines and varies from 33,400 to 292,000 mg/L. Calcium ion concentration is in the range of 2,392 to 39,280 mg/L.
SPE Members Abstract Since its introduction in Saudi Arabia in 1993, the Array Induction Tool (AIT) has been run extensively in a wide variety of petrophysical environments. These include fresh mud, salty mud, high and low resistivity formations, hydrocarbon-bearing or water-bearing reservoirs in carbonates and clastics sequences. In some examples, the AIT was also run in conjunction with the Phasor Induction (PI) or Dual-Laterolog for comparison purpose. As with any resistivity device, the AIT's primary product, an accurate Rt measurement, should not be overlooked. However, owing to the AIT's multiple vertical resolutions and depths of investigation, we found that the tool yields additional information about the reservoirs that is not possible with older induction tools. The AIT improves the estimation of Rt in cases of thin laminations, complex invasion profiles or when the borehole correction is critical (wash-out, salty mud, high resistivity). In many instances, permeable beds that are invaded are much easier to identify with the AIT than with dual depths of investigation induction or laterolog tools. A typical case is the interpretation of annul us. Finally, we show the AIT characteristic response in a thin magnetic marker. Introduction Induction logging has long been the primary resistivity log for decades. In a drastic departure from past induction tools which rely on hardware focusing to provide two induction measurements at two depths of investigation, the Array Induction tool (AIT) utilizes the raw data from a simple array of eight receivers to derive by software means a set of logs with matched vertical resolution and with five median depths of investigation. The increase in data permits for the first time, more detailed investigations in the radial direction, which can be of value in complicated invasion profiles or oil-based mud systems. Another departure concerns environmental corrections. Conventional induction logs are usually presented uncorrected for environmental effects. It is well known that borehole effect is significantly different for the deep and medium induction, especially in large boreholes. Another challenge to interpreting deep induction, medium induction and spherically-focused logs is shoulder bed and invasion effects. Because of the different physics underlying previous resistivity measurements, certain combinations of bed thickness and resistivity contrast in invaded beds can mask or cause false curve separation, and thus obscure the resistivity interpretation. The AIT logs are always presented fully borehole corrected, and optimized such that the responses are a good compromise between radial and vertical focusing. Numerous papers have been written about the AIT tool design, physics and processing algorithm, so we invite the interested reader to refer to these documents. P. 581
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