DETERMINATION OF FORMATION PORE PRESSURES INCARBONATE ENVIRONMENTS FROM SONIC LOGS

R.R. WEAKLEY

this article begins on the next page FFPETROLEUM SOCIETY OF CIM/SOCIETY OF PETROLEUM ENGINEERS PAPER NO. CIM/SPE 90-9 TI*S IS A PREPRINT - SUBJECT TO CORRECMON DETERMINATION OF FORMATION PORE PRESSURES IN CARBONATE ENVIRONMENTS FROM SONIC LOGS BY Robert R. Weakley PUBUCATION FK[GHTS RESERVED THIS PAPER IS TO BE PRESENTED AT THE INTERNATIONAL TECHNICAL MEETING JOINTLY HOSTED BY THE PETROLEUM SOCIETY OF CIM AND THE SOCIETY OF PETROLEUM ENGINEERS IN CALGARY, JUNE 10 TO 13,1990. ON OF THS PAPER 0 INVITED. SUCH DISCUSSION MAY BE PRESENTED AT THE MEETING AND WILL BE CONSIDERED FOR PUBLICATION N CIM AND SPEJOURNALS IF FILED IN WRITING WITH THE TECHNICAL PROGRAM CHAIRMAN PRIOR TO THE CONCLUSION OF THE MEETING. CT Formation pore pressure determinations from log properties in carbonate environments has always been a difficult task. They do not compact uni- formly with depth as do shales, nor is it necessarily true for the fluids to be in support of the overburden when abnormally pressured, as in the classical sand shale abnormally pressured enviroranent. Consequently, the traditional, somewhat straight forward, techniques of pore Pressure determination fromlog properties do not, as such, apply. We have developed an approach to determine formation pore pressures in carbonate enviroranents utilizing sonic velocity trends. This technique has been utilized on approximately 20 wells to date, as of this writing, with very good results. Wells analyzed have ranged in Pore pressures, at total depth, from io PPG to 18 PPG and have been as deep as 23000 feet. It has also been possible to identify pressure regressions. The approach will be illustrated in detail, and the results of its use on a number of wells will be presented and explained.Mud weights required to drill these wells will also be illustrated as well as any pressure control data such as a kick. 9-1 in just about any environment, there tends to be a mixture comprising a rock's makeup. Generally speaking, in all sands there is some shale, in all shales there is some sand, in all carbonates there is some sand and shale and so forth. Thus, when we speak of a rock as being a sandstone for instance, we are referring to its primary compositional nature. when we analyze a gamma ray lc>g in a classical sand shale environment, we look for a gamma ray response tothe right for shale indication, and to the left for an indication of sandstone. When we analyze the gamma ray in a carbonate environment, the peak responses to the right we will consider to be the result of some shaliness within the rock matrix. It will be in these intervals where we will concentrate our efforts. TH-B MMIQUE The first step is in detemining lithology tops. This is done by displaying the gamma ray and sonic logs in a one inch equals one thousand foot scale.

PETROLEUM SOCIETY OF CIM/SOCIETY OF PETROLEUM ENGINEERS

THIS IS A PREPRINT· SUBJECT TO CORRECTION

PAPER NO. CIMISPE 90-9

DETERMINATION OF FORMATION POREPRESSURES IN CARBONATE

ENVIRONMENTS FROM SONIC LOGS

BYRobert R. Weakley

ChlVrcn SCll'\llca

PUBLICATION RIGHTS RESERVEDTHIS PAPER IS TO BE PRESENTED AT THE INTERNATIONAL TECHNICAL MEETING JOINTLY HOSTED BY THE

PETROLEUM SOCIETY OF CIM AND THE SOCIETY OF PETROLEUM ENGINEERS IN CALGARY, JUNE 10 TO 13, 1990.DISCUSSION OF THIS PAPER IS INVITED. SUCH DISCUSSION MAY BE PRESENTED AT THE MEETING AND WILL BECONSIDERED FOR PUBUCATION IN CIM AND SPE JOURNALS IF FILED IN WRITING WITH THE TECHNICAL PROGRAMCHAIRMAN PRIOR TO THE CONCLUSION OF THE MEETING.

ABSTRACT

Formation pore pressure determinations from logproperties in carbonate env~rorunent.s has alwaysbeen a difficult task. They do not compact uni­formly with depth as do shales. nor is itnecessarily true for the fluids to be in support ofthe overburden when abnormally pressured, as in theclassical sand shale abnormally pressuredenv~ronment. Consequently r the traditional,somewhat straight forward, techniques of porepressure determination from log properties do not,as such, apply.

We have developed an approach to determineformation pore pressures in carbonate environmentsutilizing sonic velocity trends. This techn~que

has been utihzed on approximately 20 wells todate. as of this writing. with very good results.Wells analyzed have ranged in pore pressures, attotal depth. from 10 PPG to 18 PPG and have been asdeep as 23000 feet. It has also been possible to~dentify pressure regressions.

~~e approach will be illustrated in detail, and theres'll'ts of its use on a number of wells will bepresented and explained. Mud weights required todrill 'these wells will also be illustrated as wellas any pressure control data such as a kick.

9-1

BACKGROUND

In just about any enVironment, there tends to be amixture comprising a rock I s makeup. Generallyspeaking, in all sands there is some shale, in allshales there is some sand, in all carbonates thereis some sand and shale and so forth. Thus, when wespeak of a rock as being a sandstone for instance,we are referring to its pr~ary compositionalnature.

When we analyze a g~a ray log in a classical sandshale environment, we look for a gamma ray responseto the right for shale indication, and to the leftfor an indication of sandstone.

When we analyze the ganuna ray in a carbonateenVironment, the peak responses to the right: wewill consider to be the result of some shal~ness

Within the rock matrix. It will be in theseintervals where we will concentrate our efforts.

TBE TECHNIQUE

The first step is in. determining lithology tops.This is done by displaying the gamma ray and soniclogs in a one inch equals one thousand foot scale.

.'

In comp~essing data like this, a smoothing funct~on

need be applied t:D avoid a blur of data_ Lithologytops are then determ.1.ned by p.1.cking the p0l.ntswhere 8.1.ther the gamma ray or sonic shows a change.1.n the general trend. This process is .1.11ustratedin f.1.gu~e 1, with lithological tops indicated withthe dark horizontal 11.ne5_ The well utilized inthJS figure is 1n the Destin Dome area of the Gulfof Me>llco.

The gamma ray and sonic are then displayed in a onel.nch equals one hundred foo'[ scale, Againsmoothing may be required_ The lithology topsprevlously det:ermined are translated to thisd.1.splay. Next to this data we d.1.splay anunsmoothed version of the gamma ray, as well as anSP, resist.1.vity and conductivlty curves as infigures 2 through 6.

roughly 3/4 a barrel per hOur with a mud weight ashigh as 14.9 PPG in the hole. The bottom portionof the well experienced a pressure regression andmud weights could be reduced .

ADDITIONAL IMPORnN'l' POIH'l'

It is important to recognize which side of thesonLC log trend lines need be drawn. In manyinstances, it may be necessary to change from oneside to the other upon crossing lithology tops.

In figures 2 through 6, from close examdnation ofthe sonic response with respect to the gamma rayLntervals selected, it becomes necessary to switchfrom plotting trend lines on the right to the leftside of the sonic and vice versa_

EXAMPLES OF RRSULTS

I. Pore pressure determ~nations are poss.Lblefrom sonic velocity trends world wide.

CDNCLUS!OHS

The techniquegreat success_the results.

determiningin accurate

has been used on numerous wells withFigures 12 through 17 are some of

The key in successfullyformation pore pressures islocation of lithology tops.

2.

With~n lithological SectLons, we analyze the gammaray peaks which trend to the right ~n the shaledLrection. We've cLrcled these in fLgures 2through 6. WLth respect to these gamma ray peaksto the rLght, we draw sonlC log velocity trendlines, honoring the sonic velocities whLch corre­spond to the gamma ray intervals previouslycircled. These correspond~ng sonic velocities havealso been circled in figures 2 through 6 and thecorresponding trend lines have been drawn. Notethat in some ~nstances in figures 2 through 6 thatthe velocity trend lines appear on the left of thesonic log r and in others on the rLght. The sonicvelocity trend lines are then drawn onsemilogrithmic paper honoring lithology tops as Lnfigure 7.

,

".,

These lithology tops become recalibration pointsin this process. In this, the sonic velocity trendin one lithological sectlon is traced_ ThevelocHy trend is recalibrated by shifting the~racLng over at the l~thology change, joining thelast value of interval velocLty Ln the last litho­logical section with the first value ~n the next,This results in a continuous relative intervalvelocity profile as Ln fLgure S.

3.

,.

Creation of area specific overlays greatlysimplifies and aids in this pore pressuredetermLnation process.

Concentrating on sonic velocLty trends withrespect to gamma ray responses in a shaleydirection appears to be the key in dealingwith carbonates_

For this well we have a known formation porepressure at 21000 feet of 15.2 PPG equivalent mudwe~ght. We integrate the bulk densny log anddeterm.2.ne the overburden gradient. We now havewhat we need to solve for the pore pressureexponent and create an overlay for the area. Thishas been done in figure 10. Determination of porepressure for all formations can now be readdirectly from the overlay. This has been done withresults graphically displayed in fLgure 11. Notethat in the intervals wh~ch appear to have beendrilled under balanced, difficulties with torqueand drag were encountered. All the fonnatLonsencountered lacked perrneabLlity, other than theformatlon at 21000 feet whLch was of lowpermeability. This formation of approximately 15_2PPG pore pressure continued to flow at a rate of

Note, in figure 9.line through thecompacted sectLonfeet.

that we can draw a normal trendnormally pressured, normally

of the hole from 5400 to 8700

ACKNOtfLI!LlGE>I

The author wishes to express appreciation toChevron Services Co_ for its permission to publLshthis paper.

9-2

-- ------4000

6000

r,-~~

r---,

-.:1,~

~~

14000

15000

16000

17000

18000

19000

-=-------- 21000 .---------------

FIGURE 1

9-3

:;

-.-