US 3302706 A
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atent SEARCH ttuum assarss Patented Feb. 7, 1967 fire 3,302,706 SOIL GAS lROSPECTlNG FQR PETROLEUM Robert R- Thompson, Tulsa, Gltlm, assignor to Pan American Petroleum (Iorporation, Tulsa, 01th., a corporation of Delaware 1 N Drawing. Filed Feb. 21. 1964, Ser. No. 346,416
Claims; (Cl. 166-4) This invention relates to recovering oil from subsurface formations. More particularly, it relates to determining which of several possible formations underlying a certain area contains oil so a well can be drilled to that forma tion to recover the oil.
In the prior art many methods have been proposed for determining whether an oil producing formation underlies a particular area. Several of these methods involve obtaining soil gas samples from the soil over the area of interest and analyzing these samples for hydrocarbon content. Ofter a plot of the results ofv such a survey indicates that an oil-bearing formation lies under at least a portion of the area. The question then is one of how deep below the earths surface is the oil-bearing formation. An answer to this question is desirable in order to decide how deep to drill in order to recover the oil. In many areas it will be known that several formations underlying the area not potentially oil-bearing formations. The question is usually which of these is the one which is actually oilbearing.
An object of this invention is to determine which of several potentially oil-bearing formations below an area contains the oil indicated by a soil gas analysis survey so that a well can be drilled to this particular formation to aid in recovering the oil. Still other objects will appear from the following description and claims. 7
I have now found that the soil gases from each formationhave a characteristic ratio of various hydrocarbons- This is particularly true for a region of limited extent where the depth of each formation is rather constant. The two hydrocarbons with which I prefer to work are methane and ethane since these are generally present in the largest amounts. In order to avoid possible mislead ing results from methane generated near the surface, however, it is sometimes advisable to use ratios of higher ;molecular weight hydrocarbons, such as propane and the butanes. Surface effects can usually be avoided by taking samples from greater depth. In addition, two potential oil-bearing formations below the area of interest may have the same ratioof methane to ethane but different ratios of, say, isobutane to n-butane. In such cases it will be desirable to use the hydrocarbon ratio which is different for the various potential oil-bearing formations.
The technique is simply to determine the ratio of two particular hydrocarbons in soil gases obtained over fields .known to be producing from the'various potential oilbearing formationsunderlying the survey area. The ratio of the same two hydrocarbons is then measured in soil gases obtained over the survey area. A well is then drilled to a formation producing the soil gas hydrocarbon ratio like that over the survey area.
Soil gas samples can be taken directly from the soil in place. For example, a tube may be introduced to the desired depth and a sample of the gases present can be withdrawn. It is greatly preferred, however, to take soil samples and release gases from these soil samples by treatment with acid. After carbon dioxide is removed by an alkaline wash, the gas sample is analyzed by a method of gas chromatography. The same analysis technique should preferably be used with soil gas whatever the means by which the soil gas is derived. In the preferred gas chromatography technique, a stream of inert gas, such as nitrogen, continuously flows through a tube filled with an adsorbent material such as silica or alumina. The soil gas is introduced as a batch or slug which the inert gas sweeps through the column. The rate of travel of the various hydrocarbons varies with molecular weight so they leave the tube at different characteristic times. The quantity of each can then be determined by ionization in a hydrogen flame, by combustion followed by measurement of combustion products, or by other techniques, and the ratios of the various hydrocarbons can be directly determined. Still other techniques for measuring hydrocarbons in soil gases have been described in the past and can be used but the chromatographic technique appears to be much more accurate and is greatly preferred.
An important correlation has been noted during surveys by the proposed technique. This is a correlation between the density of oil in an oil-bearing formation and the ratio of hydrocarbons in soil gases above the formation. There is a substantially straight-line relationship between the methane-to-cthane ratio and the API gravity of the oil in the subsurface oil-bearing formation. This relationship is represented by the formula G=65-3.7R where G is the API gravity of the oil and R is the methane-toethane ratio. The API gravity of the oil from producing fields is known. Therefore, this relationship will usually be sufiicient to indicate the oil-producing formation under a particular portion of a survey area. This eliminates the necessity for obtaining soil gas samples over known fields and determining methane-to-ethane ratios of such samples. The oil densities from at least two potential producing formations must, of course, be different for this technique to be applicable.
One precaution which should be noted is that interpretation of the data should take into account unusual circumstances such as changes in the densities of oils in any formation in question, either in the survey area or the field used as a standard. in general, the formation in the standard field and in the survey area should be at as nearly the same depth and as near together geographically as possible.
Another variable which should be considered in making interpretations is the nature of the soil from which samples are taken. An advantage of my method, however, is that the nature of the soil seems to make much less difference in hydrocarbon ratios than it does in total hydrocarbon content.
Still-another factor considered important by some woi'lo ers is the time at which soil gas samples are taken over known fields. Some workers have noted differences, at least in total hydrocarbon content, between samples taken before the field is produced and after the field has produced for a few months or years. To avoid as much as possible diificulties from this factor, it is preferred that soil gas samples be taken over known fields as soon as. possible after production from the field has started.
The results of a survey over a wide area in Wyoming will illustrate use of my process. The survey covered an area below which three potential producing formations were known to exist. These were the Muddy, the Dakota, and the Minnelusa formations. The hydrocarbon content of the soil gas was high over several areas. Methane-toethane ratios were determined and plotted on a map with the'total hydrocarbon content of the gas.
Over one of the larger high areas, the methane-to-ethane ratio was also high. This ratio averaged about 12, but ranged from about 9 to about 17. The same was true of an area adjacent an old field. The old field produced from the Minnelusa formation. The API gravity of the produced oil was about 22. When the large area was drilled, a Minnelusa field was found which also produced oil having a gravity of about 22. The area adjacent the old field was also drilled and also produced from the adjacent to an existing fieldthis time a small one. The
soil gas survey indicated that this field. could be extended considerably with new production from the Dakota or Muddy according to the hydrocarbon ratios.
The existing field in this case produced from the Dakota formation. The productive area was later extended-as predicted with new wells producing from the Dakota. f the other two areas, one produces a 39 gravity oil from the Muddy formation. The other produces an oil of about 38 API gravity from the Dakota formation.
It will be noted that the gravities of the two crude oils from the Muddy and Dakota formations were almost the same. The methane-to-ethane ratios were also so nearly the same that the sources of the soil gas could not be distinguished from this ratio alone. Average values for samples over one area and 13 samples over the other gave methane-t0-ethane ratios which were very nearly identical. The variations in the analyses of individual samples, as noted above, show the importance of using averages for large numbers of samples in determining the oil gravity and therefore the formation serving as a. source for the soil gases. The same averages showed a difference of about '6 percent in the ratio of butane to isobutane. Whether a difference of this magnitude is really significant in this case is not known. The difference does illustrate, however, how some ratios may be identical over two areas while other ratios may be different. In such cases, the ratio which differs overthe two known fields should be used in determining the source of soil gases in the survey area. There will, of course, be some cases in which no detectable differences will be apparent between soil gases over two fields producing from different formations. In this case my process can be used to exclude most of the possible producing formations and narrow the possibilities to these two formations.
Plotting not only total hydrocarbon content, but also ratios of hydrocarbons on'a map of the survey area is usually advisable. The plot of total hydrocarbon contents of soil gas samples will generally be sufficient alone to indicate by a high spot or a halo, for example, where a well should be drilled. A plot of hydrocarbon ratios not only distinguishes between productive: subsurface zones closely spaced horizontally, although in different formations, but also aids to indicate the most favorable locations for drilling wells into such zones. For this purpose, the maps of hydrocarbon ratios may even. be
contoured in cases where there is some variation in hydrocarbon.ratios within small portions of the survey area.
The hydrocarbon ratios can be put to several important uses. For example, they can be used to determine whet-hero. surface soil gas anomaly indicates a possible field large enough to be of economic value, or if the surface anomaly actually can be broken down into two or more smaller merging or overlapping anomalies caused by two or more subsurface accumulations too small to be of significant economic value.
In addition, the ratios can be used to aid in interpreting soil gas results in. areas where complex and overlapping or merging surface anomalies are found because petroleum reservoirs in different subsurface formations occur closely spaced laterally.
The principal use, as noted previously, is to determine which subsurface formation will be productive within a given surface soil gas anomaly so a well can be drilled to that formation to recover the oil.
While recovery of petroleum from a well drilled in the survey area is the most probable use to which the well will be put, it is possible, of course, that the well may be drilled and used as an injection well in a gas drive, water drive, or the like to recover the oil.
Many other possibilities will occur to those skilled in the art. I do not, therefore, wish to be limited to the examples given above, but onlyby the following claims.
I claim: a
1. A method for recovering oil from a subsurface geological formation comprising analyzing soil gas samples taken over a survey area to determine the total hydrocarbon content and the ratio of two different hydrocarbons in said samples, plotting the total hydrocarbon content and the ratio of said two different hydrocarbons on a map of said survey area, determining the ratio of said two different hydrocarbons in soil gas samples taken over fields producing from formations from which production might be expected below said survey area, and drilling "a well within said survey area at a location indicated to be promising by the total hydrocarbon contents of samples taken over said survey area, said well being drilled to a formation producing a ratio of said two different bydrocarbons like the ratio determined over that portion of the survey area in which said well is drilled- 2. The method of claim 1 in which said two different hydrocarbons are methane and ethane.
3. The method of claim 1 in which the soil gas samples are analyzed by gas chromatography.
4. A method for recovering oil from a subsurface formation in a survey area in which there are at least two potential producing formations each known to produce, in nearby areas, oil having an API gravity differing from the API gravity of oil produced from at least one other of said potential producing formations, comprising analyzing soil gas samples taken over said survey area to determine the total hydrocarbon content and the methaneto-ethane ratio in said samples, plotting the total hydrocarbon content and the methane-to-ethane ratio on a map of said survey area and drilling a well within said survey area at a location indicated to be promising by the total hydrocarbon contents of samples taken over said survey area, said well being drilled to a formation producing oil having an API gravity indicated approximately by the formula G=65-3.7R in which G is the API gravity of the oil and R is the methane-to-ethane ratio determined over that portion of the survey area in which said well is drilled.
5. The method of claim 4 in which said soil gas samples are analyzed by gas chromatography.
6. A method for recovering oil from a subsurface geological -formation comprising analyzing soil gas samples taken over a survey area to determine the total hydro carbon content and the ratio of two different hydrw carbons in said samples, plotting the total hydrocarbon content and the ratio of said two different hydrocarbons on a map of said survey area, determining the ratio of said two different hydrocarbons in soil gas samples taken over fields producing from formations from which production might be expected below said survey area, and drilling two wells within said survey area at locations indicated to be promising by the total hydrocarbon contents of samples taken over said survey area, one well being drilled to a first formation producing a ratio of said two different hydrocarbons like the ratio determined over that portion of the survey area in which said one well is drilled, and the other well being drilled to a second formation producing a ratio of said two different hydrocarbons like the ratio determined over that portion of the survey area in which said other well is drilled.
7. The method of claim 6 in which said two different hydrocarbons are methane and ethane.
8. The method of claim 6 in which the soil gas samples are analyzed by gas chromatography.
9. A method for recovering oil from a subsurface formation. in a survey area in which there are at least two potential producing formations each known to produce, in nearby areas, oil having an API gravity differing from the API gravity of oil produced from at least one other of said potential producing formations, comprising analyzing soil gas samples taken over said survey area to determine the total hydrocarbon content and the methaneto-ethane ratio in said samples, plotting the total hydrocarbon content and the methane-to-ethane ratio on a map of said survey area, and drilling two wells within said survey area at locations indicated to be promising by the total hydrocarbon contents of samples taken over said survey area, the wells being drilled to different formations producing oils having API gravities indicated approximately by the formula G=653.7R in which G is the API gravity of an oil and R is the methane-toethane ratio determined over that portion of the survey area in which a well is drilled. v
10. The method of claim 9 in which said soil gas samples are analyzed by gas chromatography.
References Cited by the Examiner UNITED STATES PATENTS 2/1943 Smith 23-230 X 2,312,271 3,196,664 7/ 1965 Teal 73-23.l
FOREIGN PATENTS 578,811 6/1959 Canada.
OTHER REFERENCES Scott, R. P. W. Gas Chromatography, Butterworths;
. 1960, pp. 3.31 and 332.
ERNEST R. PURSER; Primary Examiner.