The 4 key elements in Pressure Transient Testing using Surface Measurements
By Rod Kelly |
Mon, 21 Jun 2010
This is the first installment of a 4 part series outlining the 4 keys of pressure transient testing from surface measurements.
Using surface pressure measurements for pressure transient testing applications is a technique that is under utilized in the industry today. Outside of Data Retrieval Corporation, which pioneered this technology in the 1980’s, I don’t believe there is another service company that is actively promoting and performing this work today.
WHP to BHP conversion technology is often given little consideration because Petroleum Engineering textbooks declare that it is only possible under “dry” gas scenarios (defined as <10 BBLS/MMSCFD liquid) and therefore thought not possible when any appreciable quantity of liquids are being produced. A factor further complicating this technique is a lack of high quality surface measurement devices available in the oilfield marketplace today. This lack of gauge quality serves to introduce an additional error source into the WHP to BHP process, further eroding the confidence in this technology by Operating company engineers. We also commonly see other reasons for skepticism from reservoir engineers, such as: 1. Thermal decay (decline in surface pressure) effects during a Pressure Build-up (PBU) and the inability for any commercially available software to compensate for this phenomenon thus rendering, in their eyes, surface measurements invalid. 2. Surface pressure data tends to be “noisier” which can introduce additional challenges for analysis in conventional type curve matching software packages. 3. The perception that downhole gauges are always positioned at mid-perforation during the test. Our experience tells us that less than 1/3 of gauge deployments are to mid-perforation. The vast majority of downhole gauges are suspended in the production tubing at some distance above the reservoir, subjecting the downhole gauge to liquid fall-back/re-injection, phase change behavior and generally the same phenomena that the SPIDR system senses from surface.
Performing Pressure Transient Testing from the surface holds distinct advantages over running wire and pressure gauges downhole, namely RISK and COST. If surface pressure measurements can be accurately converted to downhole conditions, there is NO VALID reason to not use surface measurements.
SPIDR Surface Well Testing got its start in a geo-pressured environment (onshore, inlandwater and offshore) where wells are normally deviated and pressures and temperatures are elevated. These factors drove the operators to riskless and less expensive pressure transient testing options. It has also gained favor in HTHP environments around the world. Another area of interest is by those operators whose reservoirs produce sour gas, CO2 and / or H2S. The WELLBORE RISK along with the TOPSIDE (PERSONNEL/EQUIPMENT) RISK associated with running gauges downhole under these conditions are big drivers towards the use of our technology, not to mention the enormous cost savings that benefit our customers.
If we start with these enormous advantages, we now are tasked with demonstrating the capabilities of this technology to match or closely track downhole pressure gauge results. In addition, when downhole gauges are routinely placed at some distance above the reservoir, it is even easier to demonstrate our distinct advantages.
1st Key
Start with a High quality Surface Pressure Gauge. A surface pressure gauge that incorporates high resolution (0.01 psi or better) along with high frequency data gathering (1 sample per second or better) capabilities are the minimum requirements. High Quality means a Quartz type transducer; we would go a step further and specify a dual-quartz Quartzdyne transducer which is known as the best transducer provider in the Oilfield today. The Quartzdyne transducer provides the thermal stability and thermal compensation that is critical for pressure transient testing applications over a wide variety of wellbore conditions. Another key component of a surface gauge is how it will be installed on the wellhead. Is it directly installed (as an integral part) on the wellhead or is it attached to the wellhead via a capillary tubing system? These are subtle differences but can play a large role when it comes to accounting for changing temperatures, both at the wellhead and in the environment (daytime/nighttime). Is the gauge certified Intrinsically Safe? Is it reliable? How large is the memory and how long can it record data without changing batteries? Can data be downloaded during the course of a test without stopping and reprogramming? Is it easily installed on the wellhead in a matter of a few minutes?
These are some of the key points to take away concerning the quality and characteristics of the surface pressure gauge you will want to use to gather the pressure data for pressure transient analysis purposes.
The next installment will cover our 2nd key of pressure transient testing with surface measurements which is the importance of employing a robust WHP to BHP conversion algorithm.
