Microsoft Word - SECTION6 Interpreting Gas.doc http://dc343.4shared.com/doc/sfmsz-Qy/preview.html HRH Limited Gas Analysis SECTION6 INTERPRETATION OF GAS MEASUREMENTS 6.1 INTRODUCTION 6.1.1 Where Does Gas in Drilling Fluid Come From? Schowalter & Hess (1999) identify four sources of naturally occurring hydrocarbons within the formations drilled. It is feasible that none, some, or all of these may be encountered in any given well. These accumulations are defined as follows: Continuous Phase In this situation the hydrocarbon exists in sufficient saturation to enable direct pore to pore continuity of the hydrocarbon column. This can occur where the hydrocarbon saturation is between 10% and 90% within the rock pore spaces. This is the only accumulation that is of commercial interest as this is the only situation where the hydrocarbon is free to flow towards production wellbores. Residual Phase In this case the oil or gas presence is as discrete droplets within the water-bearing pore space. These will usually stick to or even coat the rock matrix grains. Hydrocarbon of this type will not flow towards a wellbore. In reservoirs of this type only the water will be mobile. Such an accumulation would be characteristic of a hydrocarbon migration pathway between source and trap, or would also be characteristic of a reservoir from which production has already taken place. Dissolved Hydrocarbons At the molecular level it is widely accepted that formation waters contain significant quantities of hydrocarbons in solution. These are released from water produced by drilling or production and will tend to appear as free gas at the surface. However, this is unlikely to saturate the pore space of the reservoir in sufficient quantity to create commercially productive traps. Version 2.0 6-1 1 of 10 2012-06-18 23:02 Microsoft Word - SECTION6 Interpreting Gas.doc http://dc343.4shared.com/doc/sfmsz-Qy/preview.html HRH Limited : Gas Analysis Hydrocarbon Associated with Kerogen Insoluble kerogenaceous materials will tend to have associated free soluble hydrocarbons contained within the main organic mass. These can be released by drilling, but will not flow through the rock to enable production. Significant gas shows from highly kerogenaceous rocks can indicate the presence of a viable source rock. 6.1.2 Why Measure Gas? Gas is measured during drilling for two primary reasons. These are rig safety and reservoir evaluation. The HRH Limited gas package is designed to incorporate both measurements by performing total gas and chromatographic gas measurements. Measurements for rig safety are also performed by the drilling contractor (using an HRH Limited Drilgas system or equivalent). Reservoir Evaluation (Chromatographic Gas) Chromatography can be defined as the process of separating the individual components of a gas mixture followed by measurement their concentrations. The gas system performs the initial extraction of the gas samples from the drilling fluid at surface, their separation into individual components, and the measurement of their concentration in one continuous process. Short chain hydrocarbons of the alkane series (C n H 2n+2 ) will occur as gas at standard surface temperature and pressure. Specifically these gases are methane (CH 4 ), ethane (C 2 H 6 ), propane (C 3 H 8 ), iso-butane and normal-butane (C 4 H 10 ), and iso- and normal-pentane (C 5 H 12 ). A crude oil will be made up of a mixture of alkanes ranging from CH 4 to C 100 H 202. Rig Safety (Total Gas) Gas methane ethane propane butane pentane Conc (% in Air) 4.8% Conc. (ppm in Air) Conc (API Units) 48,000 ppm 30,000 ppm 22,000 ppm 18,000 ppm 14,000 ppm 240 units 150 units 110 units 90 units 70 units 3% 2.2% 1.8% 1.4% Table 6-1: Lower Explosive Limits of Hydrocarbon Gases The various hydrocarbon gases released by the drilling fluid into the atmosphere will ignite in the presence of oxygen and sufficient heat at the concentrations indicated in table 6-1 above. These explosive concentrations are termed Lower Explosive Limits (LEL). It is essential that spark-producing activities at the wellsite are only performed when the total gas values are significantly below these levels. The most critical is that of methane, since this is the most commonly occurring gas at surface. Hence total gas is usually calibrated and quoted in the artificial assumption that it is comprised entirely of Version 2.0 6-2 2 of 10 2012-06-18 23:02 Microsoft Word - SECTION6 Interpreting Gas.doc http://dc343.4shared.com/doc/sfmsz-Qy/preview.html HRH Limited methane. The control system employed at the wellsite is called the Permit to Work system, with all spark-producing activities classified as Hot Work. Gas Analysis 6.2 RELATIONSHIP BETWEEN TOTAL GAS AND CHROMATOGRAPHIC GAS RESULTS It is important to realise that the total gas value for any given formation is not the simple sum of the chromatographic breakdown. When making cross-checks between total and chromatographic results, an allowance must be made where heavier hydrocarbons are detected. For example, consider the following measurement: C1 = C2 = C3 = = iC 4 nC 4 = iC 5 = 100ppm nC 5 = 1098 ppm 756 ppm 456 ppm 238 ppm 221 ppm 98 ppm One's first instinct may be to sum these to establish the associated value of total gas, i.e. ΣC n = 2967 ppm This value is called total carbon when used in the Total/ENI Gas While Drilling analysis (Kandel et al 2000, section 6.5.1, QC). When gas levels are measured using the flame ionisation detector (FID) it is known that the response is proportional to the mass flow-rate of carbon through the flame (section 1.3.1). Hydrocarbon molecules having more than one carbon atom will give a greater response than a single-carbon molecule of methane. A rule of thumb that may be applied is that the measured concentration of each alkane should be multiplied by the number of carbon atoms present in that molecule. These multiples can then be summed to give a rough total gas. Continuing the example above: 1 2 3 4 5 x C1 x C2 x C3 4 x iC 4 x nC 4 5 x iC 5 x nC 5 ΣC corr = = = = = = = = (2 x 756) (3 x 456) (4 x 238) (4 x 221) (5 x 100) (5 x 98) = = = = = = 1098 ppm 1512 ppm 1368 ppm 952 ppm 884 ppm 500 ppm 492 ppm 5708 ppm This parameter is called corrected total carbon in the Total/ENI Gas While Drilling analysis (Kandel et al 2000, section 6.5.1, QC). Version 2.0 6-3 3 of 10 2012-06-18 23:02 Microsoft Word - SECTION6 Interpreting Gas.doc http://dc343.4shared.com/doc/sfmsz-Qy/preview.html HRH Limited Gas Analysis Where the difference between the summed chromatograph products and the measured total gas is significant, this could be due to the presence of contaminant hydrocarbons (for example from oil-based drilling fluids), or to a fault in the gas detection system. It must be remembered that this is a rule of thumb only. The precise relationship is more complex with consequent effects on the summed values. 6.3 Whittaker & Sellens (1987) GAS RATIO ANALYSIS The concentration of gas released into the atmosphere by the drilling fluid is affected by a range of parameters, only one of which is the quantity of gas contained within a reservoir formation. These other parameters include mud density, mud viscosity, the pore fluid pressure within the formations, the rate of drilling, the rate of drilling fluid circulation, the efficiency of the gas extraction system, etc. Total gas is always analysed as a volume in atmosphere because this represents the level of gas in the rig environment. During drilling, there may be a variety of activities being performed that can cause sparks with sufficient energy to ignite gas in the air and so cause an explosion. Chromatographic data is analysed from the point of view of establishing the nature of the fluid contained within the reservoir. To remove the unwanted effects of mud properties, ROP, etc. the relative quantities of each hydrocarbon are considered rather than the simple quantity released into the air. Alkane ratio analysis involves ratio quantification of the relative concentrations of methane 4 ), and pentane (C 5 ) present in the (C 1 ), ethane (C 2 ), propane (C 3 ), iso- and normal-butane (C gas sample. Whittaker and Sellens (1987) developed a three-parameter method to more accurately define the nature of the gas yielded from any one formation. The parameters they defined are Hydrocarbon Wetness (W h), Hydrocarbon Balance (B h ), and Hydrocarbon Character (C h ). Hydrocarbon wetness, balance, and character can be plotted onto a formation evaluation log (such as a mudlog) thus giving information on the nature of the hydrocarbons present in the well. A major advantage of the Wetness, Balance, and Character log is that it is plotted against depth, and so reveals the transition from gas to oil, and from oil to water, as the reservoir is penetrated. Since Wetness and Balance have an inverse relationship the traces move together, eventually crossing over, as the density of hydrocarbons increases. Thus change of hydrocarbon type can be evaluated by considering the numerical values of each ratio type. h , B h and C h do not conclusively prove the reservoir potential. It is important to note that W However they can be used as an aid to clearer definition of potential zones for future evaluation. Version 2.0 6-4 4 of 10 2012-06-18 23:02 Microsoft Word - SECTION6 Interpreting Gas.doc http://dc343.4shared.com/doc/sfmsz-Qy/preview.html HRH Limited Gas Analysis GAS RATIOS Character Wetness Balance 0 1 2 3 1 10 100 A B C D E F Fig 6-1: Whittaker & Sellens (1987) Gas Ratio Responses Previously, gas ratio analysis enabled the user to define only non-productive or productive oil and gas, using logarithmic (Pixler 1969) or triangular ratio plots (discussed below). The triangular plots are produced from data gathered at a single depth, and so are unable to produce a continuous log. Pixler plots can be single depth, or can be incorporated into logs as Version 2.0 6-5 5 of 10 2012-06-18 23:02 Microsoft Word - SECTION6 Interpreting Gas.doc http://dc343.4shared.com/doc/sfmsz-Qy/preview.html HRH Limited Gas Analysis depth based traces. Zones of interest can be further analysed using the older ratio plots if desired. Whittaker & Sellens and Pixler ratio plots are included in the attached example gas log. It should be noted that permeability of the formations will influence the amount of higher hydrocarbon gases that are released from cuttings into the mud stream. Tight rocks such as shales may retain a significant proportion of the larger molecules, thus affecting the results of the ratio equations. Interpretations of gas ratio data must be made only after referring to cuttings samples and drilling data to evaluate the associated rock type. . 6.3.1 Hydrocarbon Wetness The hydrocarbon wetness ratio is defined as a measure of the proportion of heavy alkanes present in the total gas sample, where Wh = As a general rule it can be said that W from light dry gas to residual oil. Wetness Ratio Value W h < 0.5 0.5 < W h < 17.5 17.5 < W h < 40 W h > 40 ( C2 + C 3 + C 4 + C5 ) ( C1 + C 2 + C3 + C 4 + C 5 ) h x 100 will increase as the hydrocarbon gravity increases Likely Hydrocarbon Type extremely light gas, poor productivity potential productive gas potentially productive oil residual oil, low productivity potential Zone (Figure 6-1) A B E F Table 6-2: Interpreting Using Wetness Ratio Only 6.3.2 Hydrocarbon Balance Hydrocarbon Balance is a measure of the relative concentrations of C C 5 present in a gas sample, where Bh = ( C1 + C 2 ) ( C3 + C 4 + C 5 ) h 1 and C 2 to C 3 , C 4 , and As the hydrocarbon gravity increases from light gas to residual oil the balance ratio falls. W and B h have an inverse relationship and when plotted on the same scale, can further improve evaluation. Eventually the curves will cross. To resolve the resultant ambiguity, the hydrocarbon character ratio C h (section 6.2.3) was developed. Version 2.0 6-6 6 of 10 2012-06-18 23:02 Microsoft Word - SECTION6 Interpreting Gas.doc http://dc343.4shared.com/doc/sfmsz-Qy/preview.html HRH Limited Gas Analysis Balance Ratio Likely Hydrocarbon Type B h > 100.0 light dry gas, unlikely to be productive 0.5 < W h < 17.5 W h < B h <100.0 productive gas 0.5 < W h < 17.5 Bh < W h productive gas condensate or high gravity, high GOR oil B h < W h productive oil 17.5 < W h < 40 17.5 < W h < 40 B h << W h nonproductive residual oil .Table Wetness Ratio W h < 0.5 Zone (Fig 6-1) A B C E F 6-3: Interpreting Using Wetness and Balance Ratios 6.3.3 Hydrocarbon Character When used together, W h and B h will resolve differences in the hydrocarbon makeup. However, when considering gas condensate or high gas/oil ratio (GOR) oil, little distinction Whittaker and Sellens developed the Character ratio, calculated is apparent. To combat this from Ch = ( C 4 + C5 ) C3 Wetness Balance Character Bh < W h 0.5 < W h <17.5 0.5 < W h <17.5 Bh < W h Hydrocarbon Zone condensate high GOR oil 0.5 < C 0.5 > C h h gas C C Table 6-4: Use of Character Ratio To Resolve Ambiguity in Zone C 6.3.4 Coal Gas Shows A special case is illustrated in zone D of figure 6-1 above. This is the response of the Balance ratio to coal gas shows. In such a situation, the gas is rich in ethane (C 2 ) but propane and the other higher hydrocarbons are absent. Character ratio therefore has a value of zero, while balance approaches infinity. 6.4 OTHER GAS RATIO PLOTS 6.4.1 Triangular Gas Ratio Plot This can appear confusing to the untrained eye, but is relatively simple to use. The plot is constructed in a series of stages. Firstly, an equilateral triangle is drawn with C 2 , C 3 and nC at it's apices, as shown in figure 6-2. The empiracally-derived ellipsoid plotted within the triangle will be used later to indicate the productive potential of the show. Each side of the triangle can now be used to plot ratios of C 2 4 /C 3 , C 3 /nC 4 and nC 4 /C 2 . Version 2.0 6-7 7 of 10 2012-06-18 23:02 Microsoft Word - SECTION6 Interpreting Gas.doc http://dc343.4shared.com/doc/sfmsz-Qy/preview.html HRH Limited Gas Analysis Fig 6-2: First Stage of Triangular Plot If each side of the triangle is taken to be length = 1, then these ratios can be found by C2 C3 nC 4 , , ( C 2 + C 3 ) ( C 3 + nC 4 ) ( C 2 + nC 4 ) This gives a value up to 1 for each ratio. Plot these points onto the sides of the triangle. The next step is to construct lines from each point to the opposite corner of the triangle (figure 6-3). The intersection of these lines defines the productivity of the show. If the intersection falls within the ellipsoid, the formation is likely to be productive. If it falls outside then the show is probably non-productive. An intersection falling near to the edges of the ellipsoid would indicate a marginal show. The ellipsoid shape is empirically derived from study of real wells. Fig 6-3: Productivity Potential Version 2.0 6-8 8 of 10 2012-06-18 23:02 Microsoft Word - SECTION6 Interpreting Gas.doc http://dc343.4shared.com/doc/sfmsz-Qy/preview.html HRH Limited To discover whether the show represents oil or gas, consider the ratios C nC 4 /C tot , where C tot is the simple sum of all the alkanes present, i.e. C section 6.5.1, QC). Gas Analysis 2 /C tot , C 3 /C tot and to C 5 . (total carbon, 1 Fig 6-4: Construction Stage for Hydrocarbon Type Plot A triangular graph is now constructed such that each side reads from 0 to 0.17. This is superimposed onto the existing triangular plot (figure 6-4). Each of the above ratios is calculated and plotted onto the relevant side of the triangle using this scale. Should the ratio value exceed 0.17 simply extrapolate the scale beyond this value and plot as normal. Fig 6-5: Defining Hydrocarbon Type Finally construct a line through each of these ratio points such that it parallels the side opposite to the C 2 , C 3 or nC 4 apex (figure 6-5). The three lines will form a triangle whose way up will indicate whether the C n /C tot ratios represent oil or gas. Version 2.0 6-9 9 of 10 2012-06-18 23:02 Microsoft Word - SECTION6 Interpreting Gas.doc http://dc343.4shared.com/doc/sfmsz-Qy/preview.html HRH Limited Gas Analysis 6.4.2 Pixler Gas Ratio Plot This is a simpler plot based on ratios C 1969). 1 /C 2 , C 1 /C 3 , C 1 /C 4 and C 1 /C 5 , dating from 1969 ( Pixler These ratios are plotted onto the vertical axes of the plot (figure 6-6). Analysis of real well data has shown that these ratios can indicate whether a formation bears oil, gas or water and whether or not it is productive. The points corresponding to each ratio are then joined together to produce a slope, shown in figure 6-6 below as a red line. If this slope is positive, i.e. increases from left to right, the show is likely to be productive. Any negative slope, e.g. C 1 /C 4 < C 1 /C 3 , suggests that the formation is likely to be non-productive or water-bearing. Generally speaking, an extremely steep slope, e.g. C the gas zone, would suggest very tight formations. 1 /C 2 low in the oil zone and C 1 /C 4 high in On the left, the above show (section 6.3.1) represents productive gas condensate (?) from a reservoir that may be considered to be a little tight. The red line crossing the oil/gas zone boundary shows the hydrocarbon to be transitional between gas and oil. The example on the right is a non-productive water-bearing zone. C 1 /C 1000 2 C 1 /C 3 C 1 /C 4 C 1 /C 5 1000 C 1 /C 2 C 1 /C 3 C 1 /C 4 C 1 /C 5 Non-Productive Gas Non-Productive Gas 100 100 65.0 Gas Gas 15.0 10 10 Oil Oil 2.0 Non-Productive Oil 1 1 Non-Productive Oil Fig 6-6: Pixler Gas Ratio Plots These conclusions were confirmed by later production tests. On the example log (and figure 6-7 below), the Pixler ratios are plotted against depth. The geologist should look for the presence of each ratio on the plot. Above the C 1 /C 2 shading is at the left of the column and is transparent. The difference between C 1 /C 2 and C 1 /C 3 is shaded as yellow, C 1 /C 3 and C 1 /C 4 difference is shaded green, and C 1 /C 4 to C 1 /C 5 difference is shaded blue. Version 2.0 6-10 10 of 10 2012-06-18 23:02