Determination of source and breakthrough mechanism of water production in a naturally fractured basement reservoir by analyzing water production data

Le Minh Vu, Tran Thai Son, Vu Viet Hung

Excessive production of water in the naturally fractured basement reservoir has been one of the most intriguing problems in production engineering. Water may appear immediately at the early production stage or breakthrough after months on stream. Available methods to determine the source of produced water are entirely based on chloride content. The stiff diagram is one of the well-known graphical methods to confirm whether it is formation water by integrated various chemical components. With the Stiff diagram, the operators are able to detect formation water in the early days, which apparently assists in quantifying the potential of scale deposit, erosion, and so on.

Apart from the water sources, it is crucial to understand the water-out mechanism also via a graphical approach. The characteristic trend of water-oil-ratio and its derivative with time in log-log plot indicates the variety of slopes reflecting different flowing mechanisms and a vast majority of them are attributed to the coning and channeling dependent on the level of pressure depletion and rock-fluid interaction. A thorough understanding of the water source and its break-through mechanism is indispensable to production monitoring and optimizing well production performance in the long run.

Keywords: Water analysis, water breakthrough mechanism, excessive water production, production management, fractured basement, graphical methods.

1. Introduction

Water production is among the problematic issues of the oil and gas industry nowadays. Formation water may appear at the beginning or after a period of production. In the early days, chloride and salinity assist in identifying the origin of water - whether it is drilling mud loss or formation water. In addition, the evaluation of formation water composition provides pertinent information to deal with potential scaling and corrosion from the exposure of formation water to well completion. During the production phase, the produced water has been analyzed constantly to monitor well performance and water encroachment so as to come up with a mitigation plan to sustain high production rates.

As mentioned, the determination of formation water from the sample analysis is fundamental along with other theoretical approaches which diagnose production data in the period with and without water production. In this paper, graphical methods are employed to get a close look at the Stiff diagram plotting produced water composition and the water-cut behavior by a hypothesis developed by K.S.Chan [1]. Those methods have the advantages of being simple, handy, and cost-effective. However, the downside is the sampling frequency, which apparently increases the sampling cost and lab experiment. The Stiff diagram and Chan's hypothesis have been widely applied in X fields, Cuu Long Basin offshore Vietnam as water production was observed from the early production phase. By diagnostic plots of the chemical indicators and water-cut with time, formation water and coning demeanor were confirmed in few wells which, indeed, supports a revision of the production regime to cope with reservoir management policy.

2. Graphical methods for produced water analysis

Most of the existing graphical methods are based on the comparison of the chemical indicators. This indicator consists of the amount of anionic or cationic necessary to add to or removed from the compound 1 mole.

C (meq/L) = [g.m- 3] xChange/Mass = [g.m - 3]/equivalentwt

Where:

C: The equivalent conversion factor

Change: Chemotherapy;

Mass: Mass mole;

Equivalent wt: Volume equivalent conversion

The equivalent conversion factor for some common ions is depicted in Table 1.

Table 1. The equivalent conversion factors[2]

Among the available textbook methodologies in produced water analysis, the Stiff diagram is the most popular one in which some key chemical indicators represented for seawater and formation water are plotted altogether as illustrated in Figure 1. In a Stiff diagram, data is plotted as a polygon, with cations to the left and anions to the right. Stiff diagrams are useful for looking at spatial relationships because they can be readily plotted on a map. In this respect, it is also a robust tool to compare different water sources.

Figure 1.A typical Stiff diagram [2]

This method is easy to construct graphs of chemical indicators of multiple samples from different sources. this diagram may point out a change of the chemical indicators from different sources in space and time. However, this method has the disadvantage that there is only one analysis performed on a single diagram.

Similarly, other methodologies being widely used in groundwater analysis include Piper, Durov, and Schoeller diagram, ion balance diagram, radial plot, and chemical properties vs. time plot[2]. Nevertheless, they are simple graphics to track changes of each ion and do not analyze the combination of ions at the same time. Zaporozec summarised the methods for the presentation of water analyses which were divided into four major sections: classification, correlation, analytical and illustrative [3].

All the above methods depend primarily on the purpose and the specific field applied. The change in the ion index of formation is complicated because water may penetrate from various sources. In the narrow scope of this research, the Stiff diagram is the basis for analyzing the water sources and their change with time.

3. Diagnostic plots to determine water breakthrough mechanism

Tracking water movement in the reservoir is a top priority in reservoir management when the wells commence water-out. There are numerous strategies for monitoring water encroachment, such as material balance, graphical analysis of production water, and simulation model. An intuitive method introduced by K.S.Chanin 1995where the WOR (water-oil ratio) and its derivatives are drawn on the log-log plot with time. The disparity between the WOR and WOR'suggestspotential issues: water channeling or coning. The formula for computing the WOR and its derivative are as follows:

Chan's hypothesis was based on the examination of actual production data to forecast the water-cut behavior with time. As mentioned, two typical issues at the excessive water production wells were attributed to the coning and penetration by channel (fractures, and aquifer zones). Figure 2plots the ideal expression of the WOR-WOR' relationship.

Figure 2. The K.S.Chan ideal plots[1]

According to Chan, a log-log plot of WOR behavior for both coning and channeling is divided into three periods. The first period extends from the start of production to water breakthrough where the WOR is constant for both mechanisms. When water production begins, during the second period, WOR behavior becomes disparate for coning and channeling. In the transition period, the rate of WOR increase after water breakthrough is relatively slow and gradually reached a constant value for the coning mechanism. For channeling, the water production from the breakthrough layers or fractures increases very quickly. The end of this period shows the WOR resumes at about the same rate. After the transition time, Chan’s plot describes the WOR increase to be quite rapid for both mechanisms, which begins in the third period. 

Figure 2 illustrated WOR time derivatives (WOR') versus time for the different excessive water production mechanisms. Chan mentioned that the WOR' can distinguish between coning and channeling. Coning shows a changing negative slope, as channeling WOR' curves show an almost constant positive slope.

The excessive water production is controlled by the reservoir management or well workover depending on the type of problem occurring at the well. The prediction of the water-cut tendency is vital to remedial treatment like water shut-off and changing production conditions. Classification of well problems and water shut-off suggestions is mentioned in [4]. 

4. Water production behavior in naturally fractured basement reservoir in X field

The naturally fractured basement reservoir in the X field was first discovered in 2008 by three wildcat wells. To date, six wells have been drilled into this reservoir in the exploration and development phases as shown in Figure 3. Well testing data from the exploration wells demonstrated the recovered water was identical to the drilling mud loss and there were no signs of formation water. Chloride levels are diminishing steadily but remained above 30,000ppm (Figure 4). However, only well X-8P produced water immediately from the inception. The chloride content was decreasing slowly to around 22,000ppm after three months on the stream as shown in Figure 4. The question is whether well X-8P has been producing formation water or drilling mud. After more than a year of production and having no water injection, the chloride content and other chemical components behave consistently and no doubt has on their origin.

Figure 3. Top X fractured basement depth map

Figure 4. Water production and chloride content with time in X fractured basement reservoir
producers

The results of produced water analysis are shown in Table2. The sample of 1.01.41, 01.53, 01.56, and 01.58 are analyzed for X-8P from the beginning. Based on Saline Water Classification, this produced water is a kind of saline water type [5].

Table 2.Water sample analysis of Thang Long fractured basement reservoir producers

As soon as water appeared in X-8P, consistent analyzing of samples was indispensable to provide adequate information for plotting the Stiff diagram. The water sample analysis result, then, double-checks with the ones of nearby fields for reference.

In Figure 5, the first water samples lay far from the formation curve of nearby fields (in orange-LS.01). Visually, the envelopes gradually shrink and move towards the formation of water (the blue and red lines). Water sample analysis dated 18/12/2014 (in orange) was almost approaching the formation water one.

In principle, it is possible to distinguish three fundamental stages in the above Stiff diagram. The shape of the envelope of cations and anions has been changing with time. Early-stage, water production was mostly concluded as drilling mud loss. The shape of the envelope tends to gradually shrink in the second stage depending on the level of water production and the distance to water sources. During the second stage, the envelopes of water production were in proximity to the formation water and the well may produce entirely formation water. The third stage appeared only when the reservoir had an influx from other water sources. The speed of moving from the second to the third stage depends on the capacity of the external water source. Usually, the indicators change in the third phase is Ca or/and HCO3.

Figure 5. The stiff diagram compares water sample analysis of X-8P well with time

This technique is similarly applied to the second well X-1P. The envelope expressed the changes identical to X-8P well. The envelope of this well, as described in Figure 6, July 2014 was pretty close to the formation water curve.

Figure 6. The stiff diagram compares water sample analysis of X-1P well with time

By statistical methods, a baseline of the formation water for this kind of reservoir is developed and serves as a pragmatic source for analog down the road. The amplitude change in each ion indicator is depicted in Figure 7 and Table 3below.

Figure 7. The baseline of formation water of naturally fractured basement reservoir in X fiel

Table 3. The amplitude changes of ions in the formation of water

Through the Stiff diagram, water production from this fractured basement reservoir was pinpointed as a kind of formation water. As a result, the well placement of the third production well, TL-2P, was fine-tuned with well trajectory kept in close proximity to the top structure and shunned potential fracture areas deemed connecting to the water zone. Consequently, the wells produced free water from the beginning and the Stiff diagram proved a good practice in production monitoring and well placement.

5. Determination of water breakthrough mechanism

After confirming the sources of produced water, understanding the breakthrough mechanism is crucial with regard to optimum production regime to level off the water cut. It is the point to bring up Chan’s diagnostic plots to analyze production data. The plots in Figures 8 and 9 are designated for wells X-8P and X-1P respectively.

Figure 8.Seismic cross-section through (A) X-8P and Chan’s diagnostic plot (B)

Figure 9.Seismic cross-section through X-1P andChan’s diagnostic plot

The water production in X-8P and X-1P appeared from the first days and was presumed to originate from the bottom aquifer. Moreover, the rapid rise of water production could be from high permeability channels (large fractures connected directly to the bottom water zone) as illustrated in the seismic cross-section. Oil production was maintained for a while until the coning influence. It can be seen that a separate mechanism occurred in both cases where the coning appeared shortly in a few months then abruptly switched to channeling.

In the X-8P case, a step change of data was recorded over 100 days which coincided with the effort to boost production by drastically applying pressure drawdown to productive fractures at the oil zone close to the well heel. After a period of severe depletion, water started channeling massively to the well, and production curtailment is expected. In contrast, well X-1P water encroachment was smoothly transitioning from coning to channeling due to low permeability in the oil zone and drastic depletion of reservoir energy. Drastic depletion has not made any difference to the well because of low average permeability from the well testing (permeability~ 1.7md).

Depending on the mechanism of water production and reservoir properties, an adaptive strategy shall be made to slow down the water-cut development. In the naturally fractured basement reservoir, transition time between coning and channeling are fairly short if producing at excessive rates. Therefore, it is essential to constantly update Chan’s plots to stay tuned with the reservoir performance, thus optimizing production.

In a broader view, more application to the fractured basement wells with longer production history and production rates is paramount to improve the accuracy of the assessment. By far, they are a much simple approach for daily operation monitoring. 

6. Conclusions and recommendations

- The graphical methods in analyzing produced water help determine sources of water production in the naturally fractured basement reservoir in the Cuu Long Basin. The shift of the envelope in the Stiff diagram was exhibited in three main phases: phase (1) drilling mud loss, phase (2) formation water, and phase (3) additional water influx.

- When determining the water-out mechanism, K.S.Chan’diagnostic plots indicate whether it is coning or channeling and the transition time if applicable;

- A general trend of water from the naturally fractured basement reservoir in X field has been developed in the hope to be a source for reference in dynamic Cuu Long basin analysis;

- The more applications the better the accuracy level of the reservoir performance monitoring by these approaches.

References

1. Echufu-AgboOgbene Alexis. Diagnostic plots for analysis of water production and reservoir performance. Master of Science in Petroleum Engineering. December 2010.

2. Erika Elswick.Methods for analytical geochemistry.Geology 214 (Lectures), IndianaUniversity.2012.

3. Alexander Zaporozec.Graphical interpretation of water-quality data.Department of Geology and Geophysics, University of Wisconsin.197; 10(2): p. 32 - 43.

4. Abbas Ali Changalvaie1, KazemLovimi, Abbas Khaksar.Mechanism of excessive water production in an Iranian offshore oilfield.Petroleum Engineering Department, Islamic AzadUniversity.2012.

5.Water sample analysis &evaluation report.Laboratory of Research & DevelopmentCenter for Petroleum Processing. December 2014.

This paper is published on PetroVietnainJournal, ISSN-0866-854X, Vol 6, Jun 2016

Link: https://www.vpi.pvn.vn/en/ViewResearch.aspx?gid=13&Id=2082