Good News and Bad News

Title: Good News and Bad News

Author: R.E. (Gene) Ballay, PhD

Publication: The Outcrop, May 2009, p. 6, 12, 25

INTRODUCTION

First, the Good News.

When observed, mud filtrate invasion likely signals that the formation has at least some amount of permeability. At the simplest level, and assuming a contrast in Rmf and Rw, there may be SP development, which in the presence of potassium feldspars, or uranium, can allow one to identify a reservoir that would not be clear on the GR, and can even offer an estimate of Rw.

At yet another level, applying Archie’s equation to the invaded zone and taking Rmf from the Log Header, there arises the possibility of deducing the “m” exponent, in the water leg (even though the local Rw is not known, a-priori). With ‘m’ in hand, one then applies Archie’s equation to the deep resistivity, and estimates the local Rw. This Rw estimate may (should) be compared to that calculated from the SP.

Next, the Bad News.

In actual fact, a number of phenomena conspire to often make an evaluation in the presence of invasion, more difficult. For example, the exact placement of the mud filtrate invasion front, with respect to the various radial tool responses, which are themselves each different, and which are being combined in calculations.

More Good News.

If petrophysics were so relatively simple as finding a water leg, deduce “m” from Rxo, estimate Rw from the Rdeep and then calculate Sw, we would find ourselves bored (and replaced by a computer). However with an understanding of the physical phenomena at play, one is often able to recognize the presence of ‘bad news’, and in many situations compensate for, or minimize, the effects.

The Ideal World and the Real World

Invasion occurs in two phases, Spurt and Filtration. Spurt invasion corresponds to exposure of freshly drilled rock to the drilling mud, and is followed by Filtration, which is mud filtrate passage through the mud cake. The distribution of mud filtrate, within the pore system and across time, is dependent upon mud type, the original formation fluids and the formation wettability.

In general, the salinity of the mud filtrate will differ from that of the formation water,and in the case of a relatively fresh mud, invading a hydrocarbon charged salty water formation (for example), fluid fronts of two salinities can arise, consisting of the fresh mud filtrate (near the wellbore) and the salty connate water. The potential for two salinity fronts, on contrast to the  normally assumed step profile, immediately challenges an interpretation of the data, and assorted calculation. Allen et al illustrate the consequances of failing to recognize an invasion annulus, provide guidelines to on when Resistivity Tornado Charts are appropriate, and discuss/ illustrate this issue in much greater detail than is done here.

George et al investigate the annulus issue in a 30 foot thick salty water carbonate, drilled with fresh water base mud, to find that artificially low, induction log resistivity, ‘pay’ can result.

As lateral movement of the front progresses (assuming a vertical well), gravity will also be at work, since in general the mud filtrate and formation fluids will have different densities. The magnitude/rate of change due to gravity depends up on the fluid density differences, and vertical & horizontal permeabilities. In today’s highly deviated and horizontal wells, gravity effects can lead to an asymmetrical distribution of the filtrate, around the wellbore.

Gravitation effects can be visualized , and quantified, in the laboratory, with time lapse CT Scans.

Finally, we should note that the absence, or change, of an invasion profile may indicate the presence of immoble oil. Proper oil-in-place calculation and/or reservoir management protocols require one to distinguish between the mobile and immobile oil (Figure 1), and an examination of the invasion profile may offer that option (the Sadlerochit, at Prudhoe Bay, comes to mind, and there are many other examples) .

LS5-2009Why perforate a tar interval? Why inject water below a tar mat, if the pressure support does not benefit the light oil production?

Permeability and Fractional Flow Estimates, Invasion, and associated effects,are like so many things that we do in formation evaluation, in that the more questions asked, the more issues that arise. That does not mean, however, that it’s an insurmountable problem or even necessarily all bad news. Ramakrishnan et al take the approach that ‘when life hands you a lemon, make lemonade,’ and proceed to regard invasion as an uncontrolled experiment, which may possibly be inverted to yield an indication of fractional flow.

C. Y. Yao recognizes that the time lapse log responses relate to permeability, and  proceeds to an estimation, which is compared against core and production.

Jesus Salazar of the University of Texas (download his Power Point file, and additional material, from the University of Texas Site) estimates permeability from core calibrated, mud filtrate based, simulations and compares that to a Winland estimate, while Sigal & Salazar publish a similar study in the Petrophysics journal.

Good News, Bad News, No News and a Good Friend

Mud filtrate invasion signals the presence of permeability.

Radioactive reservoirs (potassium feldspar, uranium effects on the GR, etc) may not be apparent on the GR, and yet display SP development and/or the presence of mud cake. In such a case, it can be invasion that allows one to recognize the presence of a potential reservoir, that otherwise may have gone unnoticed.

Ratios of Archie’s equation, in the flushed and uninvaded zones, allow an estimate of  saturation that is independent of both porosity and cementation exponent. Not only does this facilitate a QC cross-check on routine (with porosity log) saturations, but can in the case of vuggy porosity, alert one to changes in the cementation exponent.

Invasion and permeability (plus other attributes) are  related, so that the invasion profile can allow an estimation of permeability, and fractional flow.

Not all the news is good, however, in that at the simplest level, variable depths of invasion -tool radii of investigation, can confuse an interpretation. And if a salinity annulus has been formed, intervals of ‘pay’ may even be missed.

Finally, there are times when an invasion profile does not develop. No News may signal the presence of immobile oil. Why perforate a tar interval? Why inject water below a tar mat, if the pressure support does not benefit the light oil production?

Each time I look at an invasion profile, my memory goes back in time. As an Aramco petrophysicist, I receipted, QC’ed and evaluated newly acquired log data. Once satisfied with the results, they were then presented to Geology and Reservoir Management, and I remember as if it were yesterday, reviewing an analysis with my friend Yahya Shinawi (Aramco Reservoir Management). While I was discussing the porosity and saturation per routine calculations, he quickly noticed, and inquired about, an apparent resistivity annulus signature. From that I gained a lesson that I remind myself of before every presentation: Never Underestimate Your Audience. I’ve lost contact with Yahya over the years, but I wish him well, wherever he is.


Acknowledgement

It is a pleasure to acknowledge the following individuals, for kindly looking over this material. Omissions, typos etc remain, of course, my responsibility.

Allen, David
Ramakrishnan, Rama
Siddiqui, Shameem
Torres-Verdin, Carlos

 

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