Integration Of Petrophysical Log Data To Qualify And Quantify Reservoirs

Project and Seminar Material for Geology

Integration Of Petrophysical Log Data To Qualify And Quantify Reservoirs


Chapter One


1.1 Introduction

One very important aspect in exploration geophysics which will complement previous data acquisition is the information from well log data (wireline data), this does not only gives information about the petrophysical properties of the subsurface formation but it is a major tool in linking stratigraphy, delineating reservoir properties of a formation, calibrating seismic data and in correlating lithology where more than one wells are available.

Formation evaluation is the practice of determining both the physical and chemical properties of rocks and the fluids they contain. The objectives of formation evaluation are to evaluate the presence or absence of commercial quantities of hydrocarbons in formations penetrated by the wellbore, to determine the static and dynamic characteristics of productive reservoirs, detect small quantities of hydrocarbon which nevertheless may be very significant from an exploration standpoint, and to provide a comparison of an interval in one well to the correlative interval in another well. It can be performed in several stages such as during drilling by mud logging, logging while drilling, during logging (quick look log interpretation) and after logging (detailed log interpretation), by core analysis in the laboratory, etc. Wireline logs are one of the many different sources of data used in formation evaluation.

Using wireline log data, formation evaluation and petrophysical analysis gives reservoir data that can be used for future reserve estimation and reservoir analysis.


1.3 Research Purpose and Objectives

The aim of this study is to integrate petrophysical log data to qualify and quantify reservoirs in order to assess the production potential.

The objective includes;

  1. Knowing the lithology through the identification of sand units from chosen top sand to the last hydrocarbon bearing sand, using Gamma Ray Log.
  2. Estimation of shale volume and reservoir thickness.
  3. Assessment of effective porosity
  4. Determination of water saturation.
  5. Estimation of log derived permeability.
  6. Facies analysis by classifying reservoir sands and their depositional environment from the log motifs.
  7. Identification of hydrocarbon and gas-bearing sands and gas/oil contact from density log in combination with the neutron porosity log.

1.4 Scope of Study

The scope of this work borders on using suites of wireline logs, to interpret the properties of the formation and differentiate sand (reservoir) from shale (non-reservoir) by integrating other Petrophysical logs such as resistivity logs, porosity logs etc. to obtain lithologic sequence. Log cross plots such as compensated neutron log and formation density compensated log will be used to accurately determine the true formation porosity of the reservoir. Porosity determines the storage capacity for hydrocarbons and permeability determines the fluid flow capacity of the rock formation. Saturation is the fraction of the porosity that is occupied by hydrocarbons or by water. This method is also used to determine pore pressure and gas bearing zones within the reservoir. Finally, capillarity determines how much of the available hydrocarbons can be produced. Accurate evaluation of the formation are essential to access the economic viability of these reservoir wells in the Niger Delta oilfield.


1.5 Significance of Study

This study will help optimize reservoir characteristics and carry out reservoir monitoring & management of the wells.

With advent of modern well logging tools with enhanced data analysis reservoir wells cannot be over emphasized.

Therefore this study has the tendency to enhance the hydrocarbon potential of the Niger Delta basin.

It would help to carry out detailed characteristics of the minor & major solid and fluid fractions both in reservoir and in shales (containing varying amounts of clay bound and capillary bound water), in Niger Delta.


1.6 Geology of Niger Delta

1.6.1 Regional Setting

The Niger delta is a Cenozoic sedimentary basin situated on the continental margin of the gulf of guinea in the Equatorial West Coast of Central Africa between latitude 30 and 6 0 N and longitude 50 and 80E (Doust and Omotosola, 1990; et al, 1997). It is situated at the intersection of the Benue Trough and the south Atlantic ocean where a triple junction developed during the separation of the continents South America and Africa in late Jurassic (Whiteman, 1982; Obaje 2009.) it covers an area of about 75,000Sq km extending more than 300km from Apex to mouth and is composed of an overall regressive clastic sequence which reaches a maximum thickness of 30,000 to 40,000 ft. (9,000 to 12,000m). (Evamy et al., 1978; Doust and Omatsola, 1990).The sediment deposited in Niger Delta is supplied by the Niger River which is 4,100km long and rises in the mountains of Sierra Leone to West. The largest tributary is the Benue River with which it has it confluence in central Nigeria. (Shannon and Naylor, 1990). During the Tertiary, the Niger Delta built out into the Atlantic Ocean at the mouth of the Niger-Benue river system, an area of catchment that encompasses more than million square Kilometers (about 1,200,000km2) of predominantly savannah-covered lowlands. (Doust and Omotsola, 1990).The Cenozoic Niger Delta is framed by a set of older, stable mega tectonic elements. At the eastern fringe of the Niger Delta, there is a similar but complex feature, the Calabar Flank is the subsurface continuation of the Oban Massif. The Calabar Flank breaks off along the Calabar hinge Line which trends in a SE/NW direction. To the north of the Cenozoic lie the Senonian Abakaliki Uplift and the post Abakaliki Anambra basin. These latter units were also stable elements throughout Cenozoic time. (Murat, 1970; Merki, 1972). The sedimentary basin of the Niger delta encompasses a much larger region than the geographical extent of the modern Delta constructed by the Niger Benue drainage systems. It includes the Cross River and extends eastwards into the continental margins of neighboring Cameroun and Equatorial Guinea (Reijers et al, 1997). The present day Niger and Benue valleys are developed along areas of Mesozoic and Cenozoic sediments which separate the massifs exposed basement rocks. Westward from Delta is Dahomey basin, a coastal and shelf continent sediment wedge of these areas, the Niger Delta is the only province with substantial oil production (1.29 billion barrels/day in 1987) (Shannon and Naylor, 1990).


Chapter Five


Discussion and Conclusion

5.1 Reservoir Quality and Grain Size

Petroleum reservoirs are dominantly clastic or carbonate rocks. Shale or clay beds are not good reservoir rocks because they lack both effective porosity and permeability and can act as barriers to the lateral and vertical flow of fluids, thus, the inclusion of shale particles and clay minerals within a sandstone or carbonate matrix will tend to reduce the quality of the formation as a reservoir.

Therefore, the grain size of Uzek Well was inferred from the gamma ray log by its response to clay minerals / contents. Uzek Well log revealed that gamma ray values increased to the right of the track with high clay contents and fine-grain size, and decreased to the left indicating coarse-grained sands and low clay gamma ray values. The decrease in clay content was linked to an increase in the grain size and this relationship led to a direct correlation between facies and log shape, and the close relationship between the gamma ray log and sandstone grain size.


5.2 Analysis of Petrophysical Parameter Estimation

Net/gross ratio was used to define the proportion of the intervals that were considered to be reservoirs and it aided in the understanding of the formation. This ratio is unitless and reflects the overall quality of a zone not minding its thickness. These intervals indicated areas/units where sand deposition is concentrated, and where better reservoir quality is to be found with variations in the quality of sand. In attempting to distinguish net reservoirs and net pay intervals in the Uzek Well, cut-offs were used and zones which are porous and permeable were easily identified.

Gamma ray, neutron, and density logs were used as indirect indicators of permeability of the Uzek Well reservoirs because core is generally of limited extent and could not be relied on to define all net reservoir zones, hence, reliance was placed on the wire line log data due to the fact that it indicated the presence of fluid invasion by mud filtrate. Low gamma ray reading indicated low clay content and higher permeability, while high neutron density porosity indicated high permeability. The applied cut-offs were used to define the limits of the porous, permeable, and hydrocarbon producing zones in the Uzek Well and these zones were selected based on statistical analysis.
According to Rider (1986), the following tables give better explanation of porosity and permeability description of reservoirs.

Table 6: Qualitative Evaluation of Porosity

Percentage Porosity (%)Qualitative Description
0 – 5Negligible
5-10Poor
15-20Good
20–30Very Good
> 30Excellent
Table 7 Qualitative Evaluation of Permeability
Average K_Value (md)Qualitative Description
<10.5Poor to fair
15–50Moderate
50 – 250Good
250 – 1000Very Good
> 1000Excellent

 

The average water saturation revealed the proportion of void space occupied by water in the Uzek Well reservoirs based on the calculations made, and it showed that water saturation of the reservoirs are low, thus, high hydrocarbon saturation and high hydrocarbon production.


5.3 Porosity and Permeability Relationship in Uzek Well

Porosity is often related to permeability, this is particularly evident in clastics but often less predictable in carbonates. Cross plots of core porosity against permeability of the Uzek Well reservoirs show that low gamma ray value indicated low clay content and higher permeability, and high neutron density porosity indicated high permeability. Also, the correlation coefficient values obtained show a fairly strong linear relationship between the two variables in all the reservoirs. This reveals that Uzek Well reservoirs are permeable and have porosities of some form that are in communication.


5.4 Analysis of Cross Plots

Cross plots were done to verify log calculated values against those derived from core analysis data. The persistent difference between log and core porosities is an indication of the difference in their physical conditions which explains the fact that core porosities are measured under atmospheric conditions, while log porosities are measured under reservoir conditions, notably of pressure and temperature.

Cross plotting neutron and density porosity values was done to identify pure matrix and their related porosity. The cross plotting of incompatible logs was done to quantify lithology. The gamma ray log values were plotted against the neutron log values to bring out the relationships between the two. This plot shows that there is a consistent, straight line relationship between the two where both the gamma ray and the neutron logs are reacting to a shales and stone mixture. Each log shows the volume of shale in its own way. Through the straight line region, changes in porosity typically involve changes in shale content. However, in the very clean sandstones there are variations in porosity which do not involve shale and the relationship between the two logs changes. The sands in the Uzek Well reservoirs are gas and oil filled and the changes in porosity affect the neutron log considerably, diminishing it as porosity increases. On the gamma ray alone, these changes are not seen but when the logs were plotted together, the relationship became evident.

On the same plot, at higher gamma ray and neutron values, there is also a relationship break due to organic matter. Since the neutron tool reacts to all hydrogen present, it reacts to the hydrogen combined with carbon in organic matter. Thus, while the gamma ray values diminish as the organic matter replaces the shale; the neutron values increased or remained high. The neutron-gamma ray plot is very useful particularly in analyzing shale changes in general.


Conclusion

The characterization of the Uzek Well reservoir sand bodies was made possible by the careful integration of well log responses and core information. The study examined the vertical sequence of lithologies of the sand bodies, trend of data, and log interpretation.

A detailed petrophysical parameter estimation of the Uzek Well showed that:

  1. Reservoir quality was found to be strongly influenced by grain size.
  2. In reservoirs where porosities are high permeabilities are equally high and vice versa.
  3. These high values of porosities and permeabilities are attributed to the well sorted nature of the sands.
  4. Porosity and permeability increased with increasing reservoir quality.
  5. Average water saturation values range from 12 to 54, while the average hydrocarbon saturation values range from 35 to 94.
  6. The formation is medium – coarse grained, well to well sorted sand that occur in upper shoreface environment.
  7. Quantitative porosity verification shows good correlation between log and core porosities. The discrepancies existing in cross plots are due to the heterogeneities of the formation and to the fact that the core data are from spot sample measurements, while log represent an average and continuous measurement.

Integration Of Petrophysical Log Data To Qualify And Quantify Reservoirs


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Integration Of Petrophysical Log Data To Qualify And Quantify Reservoirs


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