Hi Patrick,
Sorry to be tardy getting to your questions. I can not address your questions thoroughly but I'll try to provide you with some general and basic principles/rules of thumb.
can you get pressure and still not have gas flow?
Usually, with all other things being equal, high pressure means:
- Higher reserve numbers.
- Higher reservoir energy levels and higher recovery factor.
- Higher well production rates.
- Greater risk in drilling and completion operations.
For hydrocarbons to flow from the reservoir and into a wellbore, the reservoir rocks must have permeability. Usually, the greater the porosity of a reservoir rock, the higher the permeability. But, there are many cases where porous rocks have very low permeability (Eg. shales) and even though high pressure hydrocarbons may be present, they may not flow at economic rates.
what do the logs tell us exactly?
There are two general types of logs, open hole logs (run after drilling and before casing the well), and cased hole logs (run in cased wellbores).
Open hole logs are run to enable a calculation of the following characteristics of the formations penetrated by the wellbore:
- Formation tops.
- Formation gross thicknesses.
- Formation dip.
- Porosity.
- Mineralogy including clay contents and types.
- Fluids present in the pore spaces.
- Net pay thicknesses.
- Formation Permeability (not generally reliable).
- Cross-Sectional geometry of the wellbore (It isn't always circular!!)
Open hole logs are carried out primarily to determine the parameters required to make a volumetric estimate of reserves. To conduct these log calculations certain assumptions are required. As in most calculations, garbage in generates garbage out. In heavily drilled areas, good assumptions will usually be possible so the calculations will be quite reliable. But, when drilling a wildcat well, good "assumptions" will be more difficult and calculated results may be less reliable. When log interpretations are difficult\inconclusive, drill stem tests will often be carried out to assist with or confirm the log evaluations. But, drill stem tests are not always practical due to safety and other issues.
Cased hole logs are used to assist in completion operations, remedial workovers or reservoir performance evaluations. Cased hole logs can help with:
- Casing inspection \ damage appraisal.
- Cement bond evaluation.
- Identification of perforation intervals.
- Identification of casing collars and centralizers.
- Evaluation of formation porosity (not real accurate).
- Evaluation of fluid interface movement (in the formation).
- Determination of perforations contributing production.
- Determination of fluid types being produced from different perfs.
...can't the logs also tell us if there is gas present?
Well, we can make calculations from the log data but sometimes these calculations are not reliable (Eg. In a wildcat we may not know the salinity or resistivity of the brines in the formation so our calculated fluids depend somewhat upon our assumptions. Or perhaps the hole "caved in" during drilling operations and so neither the measurements generated by the logging tool nor the equations used to compute the estimate are accurate.).
how do they (in lay terms) determine the proven reserves? Do they assume the entire structure is the same as where the well hole is,...
Most lay people incorrectly believe that a high production rate means a big reserve. It doesn't. A high initial production rate just enables a more rapid depletion rate of whatever reserve may be present. Initial production rates tell you NOTHING about the size of the reserves. (Long term production rate histories can be useful for estimating reserves in mature fields, however.)
There are three basic kinds of reserve estimates - Volumetric, Material Balance and Decline Curve. The first and least reliable estimates made will be volumetric. After some production history more reliable estimates, Material Balance, are possible. These estimates will normally yield reliable reserve estimates only after 5-10% of the reserve has been produced. Decline Curve analysis can be used on very mature (50%+ depleted) reservoirs to obtain reliable reserve estimates.
If from seismic and/or geological studies you can estimate the areal extent of the reservoir (acres), and if from well log data you know the thickness of the pay zone (feet), the porosity of the rock (%), the type of fluid (%), and if from other information\measurements you know the reservoir pressure (psia), and the reservoir temperature (degrees F) and the formation volume factor of the hydrocarbons (fractional), then you can ESTIMATE a volume of hydrocarbons in place in the reservoir. Then if you have detailed analysis of the hydrocarbons you can estimate shrinkage loss due to removal of impurities and byproducts and if you have an analog reservoir or other studies you can predict the ultimate recovery factor, and voila!! You can come up with your bottomline estimate of recoverable marketable reserves. This is a Volumetric Estimate.
To the extent that the estimator has confidence in the data and the economic producibility of the reserve, a portion will be classified as proven. Some additional portion may also be considered probable while the remaining reserve, if any, would be ranked as potential. Public companies must report reserves as per requirements imposed by the exchanges but reserve classification remains quite subjective.
Following the drilling of a discovery well, a production test is performed. Pressures of the reservoir are measured both before and after the test. The test information enables a Material Balance Estimate of Reserves. BUT, the reserve computed by this type of test is only reliable if the reserve is very limited in areal extent and is significantly depleted by the test!!!
Also, how long after the final test does it take to calculate the proven reserve?
There are so many factors to consider that a general answer is impossible. Suffice it to say that the bigger the reserve, the longer it takes to calculate a reliable proven reserve.
Do you happen to know how many feet of pay we have here, and is it relatively a large amount?
The News Releases indicated a gross pay interval of about 1,200 feet, I believe. That number is VERY LARGE but there has not yet been any indication of what the net pay may be.
Hope the info herein is of some use to you, Patrick.
Later,
grayhairs |
