Evaluation of Carbonate Formation using both Gas While Drilling and Nuclear Magnetic Resonance Log (Case study, An Oilfield Southern Iraq)

Two wells close to each other (Well A and Well B) have been selected from the same oilfield in southern Iraq. Both wells were drilled through the Mishrif Reservoir, however, perforated within different sub-units. It was observed that one of the wells shows obvious flow naturally while the other did not notice any kind of flow. The type of hydrocarbon interpretation by Gas while drilling method (GWD), the results confirmed that both of these wells were located in oil-bearing reservoir with comparable gas profiles. However, petrophysical properties of the pore distribution and permeability using Neutron Magnetic Resonance (NMR) data in Techlog identified key differences in rock quality between the wells across the perforated intervals. Reservoir quality reveal very wide differences in performance in spite of both wells are penetrating the oil bearing layers within Mishrif reservoir.


Introduction
Stratigraphic column is very necessary to indicate the units and show the upper and lower boundary of this important reservoir of the Late Cretaceous (Cenomanian -Turounian) is important carbonate reservoirs in Iraq and the Middle East, it provides the reservoirs for many oilfields like West Qurna, Buzurgan, Amara, Halfay, Majnoon, Rumaila, Zubiar and Qaraf, which contain about 30% of total Iraqi oil reserves [1], [2], [3]. The aim of this study is to evaluate carbonate formation by GWD TM method which is using gas while drilling data to estimate type of hydrocarbon in the reservoir and using NMR to evaluate pore size description and permeability which is one of the main evaluation parameters for determining the potential production of a reservoir because it clarify the capability of fluids to flow through rocks.
During Exploration and Development well drilling, gas data measurement (gas shows) is common procedure. Continuous gas monitoring demonstrates the presence of hydrocarbon bearing intervals in general but rarely helps to identify the fluid types (oil, condensate and/or gas, water). In latest days, research on mud gases has proceeded to include isotopic identification of various hydrocarbon components [4].
Furthermore, determining gas origin and maturity can reveal differences in gas charge across adjacent reservoir intervals, allowing reservoir continuity to be analyzed [5].
Gas readings are used with reference to the background level. The gas readings are then graphically shown on either the mud log, or the geologist mud log. This makes an easy assessment evaluation of the recorded relative amounts of gas.
The use of Pixler and/or Geoservices (or equivalent), Wetness, Balance, Character, and Gas Normalization diagrams was likely to be limited until a few years ago [6], [7], [8], [9]. Recent advancements in gas acquisition technologies, as well as the most recent GWD TM method, allow reservoir interpretation to be done in nearly real time for fluid detection and contacts (OWC, GOC, etc.), lithological changes, and efficiency barriers, allowing operations to be managed (e.g., coring, wireline logging and sampling, testing). Traditionally, hydrocarbon abundance analyses in drill mud gas have been used to assist wireline (LWD and MWD) log analyses to define pay zone during drilling.
An advance useful tool to assessment permeability is the Nuclear Magnetic Resonance (NMR) tool. It is measuring the relaxation time of the hydrogen nucleus when excited by an external magnetic field. The relaxation time is directly related to pore size. Large pores have long relaxation times and small pores have short relaxation times. Moreover, porosity, clay-bound water (CBW), bulk volume irreducible (BVI) and bulk volume movable (BVM) can be provided from NMR logs [10]. The NMR measurement of T2 at any given depth in the well is not a single number, like R t , but a distribution over time of a series of values that gives the analyst a picture of the porosity and the pore-size distribution ( Figure: 1) [11].

Gas While Drilling
The monitoring of gas shows in circulating drilling mud was implemented with two aims in the early days of mud logging measurement. First as a safety device to indicate good behavior for drillers and second as an indication for hydrocarbon bearing zones. Today, gas shows measurement is routinely acquired in the petroleum industry for the same purpose, but it is rarely used to its full potential, due in part to the ongoing bias that the data are not indicative of formation fluids and/or that the analysis of such data is highly affected by varying drilling parameters [13].
The GWD TM methodology is based on measuring the gas concentration in circulating mud and following to quality control processes and criteria to acquire reliable data.
This methodology is based on the calculation and analysis of many gas ratios acquired by continuously collecting and analyzing various gas components (C1-C5) from the circulating mud and monitoring them at the rig site by Mud Logging Company [14].
The methodology based on a very rational and regular approach to gas shows in conjunction with drilling and mud logging, and it is integrated with additional techniques such as wireline logs, PVT/thermodynamics, and geochemistry.
In the GWD TM method, the raw gas data are:  C1, C2, C3, iC4, nC4, iC5 and nC5 expressed in ppm (parts per million).  Reduce any uncertainties following conventional logging analysis.
We perform data quality control (QC) before making any interpretations. Some dedicated gas ratios are used to assess the reliability of the gas data. A comparison can be established between the Total Gas Detector and the Gas Chromatograph readings (Theoretical Total Gas) computed by formula 1.  The other data has been used in this case study wireline log, which is consist conventional wireline log and NMR log, so before a formation is logged with an NMR tool, the protons in the formation fluids are randomly oriented. When the tool passes through the formation, the tool generates magnetic fields that activate those protons.
First, the tool's permanent magnetic field aligns, or polarizes, the spin axes of the protons in a particular direction. Then the tool's oscillating field is applied to tip these protons away from their new equilibrium position. When the oscillating field is subsequently removed, the protons begin tipping back, or relaxing, toward the original direction in which the static magnetic field aligned them.21 Specified pulse sequences are used to generate a series of so-called spin echoes, which are measured by the NMR logging tool and are displayed on logs as spin-echo trains. These spin-echo trains constitute the raw NMR data [12].

NMR Processing Workflow:
Petrophysical processing is done in Techlog software using a deterministic method.
Workflow followed is summarized below:

Figure (4) shows a compact composite of the caliper log from two different tools.
Wherever the caliper was reading less than 8.5 in, there is a bad hole flag where the bulk density and microresistivity become unreliable.

Calculated total porosity using conventional open hole data.
Porosity as an essential reservoir property can be calculated indirectly from Gamma-Gamma-Density log, Neutron log, and Acoustic -/Sonic log, in this case study density and neutron has been used, Figure (5). Because, in most status, the rock matrix takes more than two-thirds of the volume sensed by the logging tools, the conventional forms of log analysis leave many questions unresolved. The analyst does not constantly know the accurate grain density of the matrix or the resistivity of the water(s) in the pore space. Because there was no tool in conventional open hole logging that directly exposed porosity or oil, it was fair to say that the loggers measured everything they could, subtracted it from the universe, Other ratio used in this project is C1/C2 ratio that illustrates in Figures (10) and (11)

Wireline Log Data Interpretation
As per open hole wireline log data (Conventional and advance (NMR)) correlation (Figure 7), there is similarity petrophysical properties over Mishrif formation in this case study. Open hole interpretation result in MHF4 shows water saturation (SW) and porosity are very similar as well as the pore size distribution from NMR log present similarity, however well A shows a bit more larger pore size as per   o MHF4 unit as per density and neutron log shows the porosity is high about 20-25%, water saturation is low about 10-15% and regarding to NMR bins which is indicator to pore size description the comparison between this unit and same unit in the well A, the pore size description seems in the well A (which is not perforated) better than Well B (which is perforated this layer but not flowing).

Conclusion
o MHF3 unit it seems bad quality whence porosity, SW and por size description and it is as one layer but compared with same unit in well A it is divided into two part which is the lower part better rock properties than the lower part same unit in Well B.
o MHF2 unit it seems bad quality in both wells, same as properties in Well A.
o MHF1 unit it seems as two part, the lower part lower quality than upper part which is shows high porosity about 25%, l0% SW and regarding to NMR bins the big pore size is dominated which is indicator to good permeability but didn't perforate.