US3409092A

US3409092A - Method for determining mud weight requirements from bulk density measurements of shale cuttings

Info

Publication number: US3409092A
Application number: US609884A
Authority: US
Inventors: Eugene H Doremus
Original assignee: Gulf Oil Corp
Current assignee: Gulf Oil Corp
Priority date: 1967-01-17
Filing date: 1967-01-17
Publication date: 1968-11-05
Anticipated expiration: 1985-11-05

Description

Nov. 5, 1968 E. H DOREMUS 3,409,092

METHOD FOR DETERMINING MUD WEIGHT REQUIREMENTS FROM ULK DENSITY MEASUREMENTS OF SHALE CUTTINGS Flled Jan. 17, 1967 2 Sheets-Sheet 1 FIG.

WEIGHT-PPS INVENTOR EUGENE H. DOREMUS Nov. 5, 1968 E. H. DOREMUS 3,409,092

METHOD FOR DETERMINING MUD WEIGHT REQUIREMENTS FROM BULK DENSITY MEASUREMENTS OF SHALE CUTTINGS Flled Jan. 17, 1967 2 Sheets-Sheet 2 FIG. 2

Puma QZ wDOIh rhauo auuc osusnv 0F SHALE CUTTINGS INVENTOR EUGENE H. DOREMUS United States PatentO 3,409,092 METHOD FOR DETERMINING MUD WEIGHT RE- QUIREMEN TS FROM BULK DENSITY MEASURE- MENTS OF SHALE CUTTINGS Eugene H. Doremus, Morgan City, La., assignor to Gulf Oil Corporation, Pittsburgh, Pa., a corporation of Pennsylvania Filed Jan. 17, 1967, Ser. No. 609,884 8 Claims. (Cl. 175-50) ABSTRACT OF THE DISCLOSURE A method of determining the changes in mud weight re quired when drilling into or through abnormally high pressured shale which comprises taking bulk density measurements of the cuttings on site and relating the changes in shale bulk density to the required changes in mud weight.

This invention relates to a method for determining mud weight requirements from bulk density measurements of shale cuttings, especially with relation to drilling wells through regions of abnormally high pore pressure shale.

In certain areas of the earth where oil is found, of which one example is the US. Coast of the Gulf of Mexico extending from the Rio Grande River to the delta of the Mississippi River, there are subterranean layers of shale known variously as low resistivity shale, low density shale, low sonic velocity shale, abnormal shale, or most generally, trouble shale. These shales are characterized by an abnormally high pressure of fluids within their pores. These high pressures are thought to have been caused by abnormalities that occurred during the formation and compaction of the layers which resulted in the fluid in the layers supporting an excessi've part of the overburden weight. The fluids may be natural gas, oil, or salt or fresh water. When drilling a well through such a high pressure shale many problems are encountered, the most serious of which is the possibility of a blow-out. Other problems include salt water flows, mud contamination, and stuck drill pipe. A blow-out is an accident wherein the high pressure encountered within the formation literally blows the entire column of drilling mud out of the hole. If the driving force is petroleum or gas or a combination of these, a fire often results which is destructive of equipment and even human life. Even if no fire results, a blowout is very expensive in terms of the damaged or destroyed drilling rig and other equipment, and possible harm to the drillers, as well as the possibility of the well caving in which results in a lost well.

The presence of a region of high pressure shale evidences itself by gas-kicks, salt water flows, change in drilling rate, and formation sloughing which may result in stuck pipe.

The present invention provides a method of predicting the required increase in mud weight or density as the well penetrates the trouble shale, and before the penetration in said region is so deep that it is too late to correct the mud weight. With prior methods it was necessary to run a log of the well, which required that the well be drilled. With the present invention, the high pressure shale is identified as the well is being drilled, which helps the driller both in preparing to deal with the trouble shale, and also aids in selecting casing points. Also, knowing the pressure-depth relationship of the well aids generally, for example, in reservoir evaluations, drilling, and completion. An associated problem is dealing with the return to normally pressure shale after the well has passed through the layer of trouble shale. If this return is not 3,409,092 Patented Nov. 5, 1968 ice recognized, the increased mud weight, needed to safely drill the high pressure shale, could cause lost circulation in the normal shale. The invention also provides for the recognition of the return to normally pressured shale to inform the driller to take appropriate action. Also, by decreasing mud weight, drilling rate can be increased.

Some practices used in drilling high pressure shale, which are replaced in whole or in part by the method of the present invention are as follows:

When a gas-kick is first encountered mud weight is increased until the gas stops kicking. This method is eifective, but has several disadvantages including the fact that the gas-kick is not encountered until the hole is into the high pressure shale, and may be so deep into it that it may be too late to prevent a blow-out. Also it is necessary to overincrease the mud weight which is expensive in terms of the cost of the mud additives, and also could cause a fracture up-hole from the region of the high pressure shale. Another disadvantage is that it is necessary to stop drilling and lift the drill pipe off the bottom of the hole to permit free circulation of the mud with the additives therein.

Another method entails running resistivity logs and observing a change in gradient toward lower values of resistivity to indicate the presence of high pressure shales. The disadvantage here is that the hole is well into the high pressure shale before the well is logged.

A third method of handling high pressure shale is to reduce the rate of drilling. This suffers from the obvious disadvantage of the increased cost of drilling the well.

With the method of the present invention, the driller is given early warning of the trouble shale, long before a gas kick would have occurred.

In the accompanying drawing forming a part of this disclosure: FIG. 1 is a graphical representation of changes in bulk density and required mud weights; and FIG. 2. is an example of a curve of the bulk density readings taken during drilling of a well.

It is known that the bulk density of normal shale increases with depth by a regular amount. High pressure shales are characterized by a decrease in bulk density with increased depth. By measuring the bulk density of the cuttings at the site, and by the use of certain relationships relating the decrease in bulk density to mud weight requirements, the weight of the mud can be increased before the hole has gone very deep into the high pressure shale. If the well is to be drilled through the high pressure shale, the method of the invention informs the driller of the return to normal pressure. One corrective action he can take would be to set casing through the trouble layer and continue to drill with reduced weight mud.

As used throughout this application, the term high pressure shale will be understood to mean any shale or sand-shale sequence in which the pore-fluid pressure is greater than approximately 0.465 times the depth of the hole. The steps of the invention in detail are as follows:

The driller has the curve of FIG. 1 before he starts to drill the new well. The curve shown in FIG. 1 was prepared from data obtained from the drilling of a number of wells in Southern Louisiana. The curve of FIG. 1 can be used as a starting point when drilling in high pressure shales in other areas, it being understood that it may be necessary to slightly modify the curve depending upon the peculiarities of the other area. The manner of doing this will become apparent in the following discussion.

There are several ways in which a curve like the curve of FIG. 1 can be obtained and which can also be utilized to obtain data to adjust or modify the curve of FIG. 1 for a region having high pressure shale characteristics dif- 3. ferentfrom those found in the Southern Louisiana area.

One way this can be done is to directly generate the curve from data taken during drilling of a well. In this case, the driller would overcome'gas kicks, water flows, and the like by simply increasing mud weight until the problem is alleviated. If data is kept on these increases in mud weight and measurements of bulk density are taken while this is done, then a curve like the curve of FIG. 1 can be generated directly, or that data can be used to modify the curve of FIG. 1. It should be noted however, that the above outlined procedure does not constitute the method of the invention since no attempt is made to use the data to predict mud weight requirements for the well being drilled. Further, the method outlined above is not usually performed on the site, the cuttings being transmitted to a laboratory for purposes of taking bulk density measurements. This time delay would be intolerable to predict mud weights.

Another way of generating a curve like the curve of FIG. 1, or obtaining data to check or modify the curve of FIG. 1, is by the use of other logs run in wells adjacent the site of the new well, where it is expected that the new well will traverse the same or similar high pressure shale encountered by the existing well. In such cases, there is usually no bulk density data available for the existing well. One of the factors controlling the density of shale is the variation in fluid content for a unit volume of shale; an increase in the percentage of fluid, usually salt water results in a decrease in bulk density and vice versa. The increased fluid content also effects other parameters: It causes an increased conductivity and a decreased sonic velocity. There is substantial correlation betweenthe degree of variation of each of these three parameters, density, sonic velocity, and conductivity, and changes in the fluid content of shale. It has been found that there is a substantially direct correlation between abnormal changes in conductivity or sonic velocity encountered in the high pressure'shale and changes in the bulk density in the high pressure shale. A curve relating conductivity or sonic velocity to depth for the existing well is obtainable from logs of the existing well. By measuring the changes in these parameters for the existing well, and using the known values of mud weight which were used in drilling the existing well, and by relating the changes in these parameters to known values of bulk density for the normal shale, a curve like the curve of FIG. 1 can be generated.

In cases where formation pressure measurements have been made in the existing well, then that data can be used by relating it to sonic velocity or conductivity logs for that well, and proceeding as above.

In the case where a first well is drilled in a new area where there are no other wells, or in the case Where it is anticipated that the adjacent wells did not encounter the formations it is anticipated the new Well will encounter, then the simplest procedure would be for the driller to use the curve of FIG. 1 as a first approximation and modify it as he drills by means of getting new points during drilling. Such new points would be obtained from the required increases in mud weight necessary to overcome gas kicks, etc., and related to measured changes in measured bulk density.

Exploration information such as seismic surveys and gravity surveys can be helpful. Seismic surveys can help locate abnormally pressured shales because these zones are often diapiric intrusions and as such do not produce any reflections. The lack of reflections could indicate other situations, but they at least alert the driller to the possibility of the existence of trouble shale. The low density of high pressure shale could appear on a gravity survey as an anomalous low gravity area. This would also alert the driller to the possibility of the existence of trouble shale.

Once supplied with the curve of FIG. 1, the driller proceeds to drill the new well, and periodically takes bulk density measurementsof the-cuttings produced. As the.

well traverses the normal shale, a trendline, indicated by the bracketed region 10 on FIG. 2, is established for the new well. At some point, which is the beginning of the high pressure shale, the bulk density of the cuttings decreases. The trend line is extended, as indicated by dotted line portion 12. As soon as the trend line'is established, or it is clear that the well is beginning to enter the'high pressure shale, measurements are taken from the curve of FIG. 2 from the actual point representative of bulk density over to the leftto extended portion 12, and this measurement indicates the difference in bulk density. With this difference the driller refers to the curve of FIG. 1 which directly gives him the mud weight increase required for that much-of a change inbulk density. The use of two curves is by way of example only. Any equivalent technique, such as transparent overlays or the like, are included in the scope of the invention. I

There are several procedures which must be followed in the actual taking of the bulk density of the cuttings. The cuttings collected from the shale shaker must be thoroughly washed until free of all kneaded, swelled, or otherwise soft shale and/or mud and until only firm shale remains. Care must be exercised in this step because if the shale is handled excessively While being washed, some of the firm shale will become soft, and in fact, if this is carried to an extreme all of the cuttings can be reduced to soft unusable shale. The washed, firm shale cuttings must be drained and then surface dried. Rolling or blotting with paper towels has been found satisfactory.

It is important that the density measurements be made as soon as possible after the washing and drying to avoid any internal drying or decomposition of the shale which would destroy meaningful conclusions. An important advantage of the method of the invention is that it can be carried out at the drilling site. When samples are packed and carried to a laboratory off the drilling site, it has been found that the cuttings deteriorate by air dryin g or taking on excess water, or in other ways.

Any suitable method can be used to actually take the bulk density measurement of the cuttings. The pycnometer bottle method involves weighing the sample with and Without a known specific gravity liquid and then calculating the bulk density by known formulae.

The density gradient liquid column method involves a mixture of two miscible liquids of different specific gravities to produce a blended liquid of gradiated specific gravity. This mixture is put in a graduated cylinder to give a straight line gradient of specific gravity which may be calibrated by use of beads of known specific gravity. The samples are then allowed to sink into the column until they stop at some level which level directly gives their bulk density.

The Jolly balance is especially designed to determine the specific gravity of solids. The weight of the samplein air and the weight of the sample in Water are quickly determined with the instrument. The apparent loss of weight in water is the weight of the bulk of water displaced by the bulk of the sample. The weight of the sample in air is divided by the weight of the equal bulk of Water.

The Le Chatelier specific gravity bottle method is not satisfactory for on-site work because of the problem of cleaning the relatively small necked bottles that are used. Another method utilizes a series of bottles filled with incrementally increased specific gravity liquids through the desired range. The cuttings are dropped into the bottles until one bottle is found where the cutting just begins to float.

The invention encompasses any method or technique for determining the bulk density.

Various other characteristics of high pressure shale have been investigated to find whether or not they can be utilized in the method of the invention to predict mud weight requirements. The mineralogical analysis of cut tings in some wells do show a change in mineral assemblage associated with the high pressure shales. However, these differences are slight, cannot be readily obtained on site, and do not have any particular correlation to formation pressure.

Faunal assemblages can be used in some cases to delineate the high pressure shale interval, but like the mineral assemblage, do not have a correlation to formation pressure and therefore cannot be used to predict mud weight requirements.

Other shale properties, such as cation exchange capacity and grain density, cannot be used to distinguish normal pressure from high pressure shale.

0f the three parameters that are used to estimate formation pressures, density, sonic velocity, and conductivity, shale density has the advantage, for reasons which include that it is the simplest to measure while drilling is in progress. It offers improvementspover present trouble indicators in that it not only gives early warning of an abnormally high pressure zone and offers means to determine the degree of pressure increase; but will also indicate when the pressure returns to normal.

While the invention has been described in some detail above, it is to be understood that this detailed description is by way of example only, and the protection granted is to be limited only within the spirit of the invention and the scope of the following claims.

I claim:

1. A method of determining mud weight requirements when drilling a well through high pressure shale which comprises the steps of preparing a first graphical relationship of mud weight required and changes in bulk denof the shale difierent from normal changes in bulk density, taking bulk density measurements of the cuttings as the well is drilled, using said bulk density measurements to make a second graphical representation of the normal increase in bulk density with depth to establish a normal trend line of the normal increase of bulk density, extending said normal trend line through the region of high pressure shale on said second graphical representation, and using the difference in bulk density between cuttings taken from the high pressure shale and the bulk density predicted by said extended portion of said normal trend 6 line to determine the mud weight required from said first graphical relationship.

2. The method of claim 1, wherein the bulk density of the shale cuttings is determined by the gradient liquid column method.

3. The method of claim 1, wherein the bulk density of the shale cuttings is determined by the pycnometer bottle method.

4. The method of claim 1, wherein the bulk density of the shale cuttings is determined by the Jolly balance.

5. The method of claim 1, wherein the shale cuttings are washed and surface dried before the bulk density measurements are made.

6. The method of claim 1, wherein said first graphical relationship is determined from conductivity logs of wells adjacent the Well being drilled.

7. The method of claim 1, wherein said first graphical relationship is determined from sonic logs of wells adjacent the well being drilled.

8. The method of claim 1, wherein said first graphical relationship is directly generated by drilling an experimental well, taking bulk density measurements of the cuttings, increasing mud weight until any evidences of high pressure shale are overcome, and directly generating said first graphical relationship from the records of increases in mud weight and decreases in bulk density made during drilling of said experimental well.

References Cited UNITED STATES PATENTS 2,214,674 9/1940 Hayward 73-153 2,302,996 11/1942 Lilligren 73-153 X 3,368,400 2/1968 Jorden et al 50 X OTHER REFERENCES Wallace, William E., Jr., Will Induction Log Yield Pressure Data? In Oil & Gas Journal, 62 (37), Sept. 14, 1964, pp. 124-126. TN 860.039.

STEPHEN J. NOVOSAD, Primary Examiner. I. A. CALVERTS, Assistant Examiner.