Pulse Neutron Neutron (PNN) tool logging for porosity Some theoretical aspects

Transcription

1 Pulse Neuton Neuton (PNN) tool logging fo poosity Some theoetical aspects Intoduction Pehaps the most citicism of Pulse Neuton Neuon (PNN) logging methods has been chage that PNN is to sensitive to the boehole. By this, it is usually meant that the esponse of neuton logs is influenced excessively by such factos as boehole size, type of fluid in the boehole o the position of the logging instument in the boehole. To these, can be added the advese effect of mudcake in open-hole logging and cementing in cased-hole applications. The best measues fo alleviating these vaious difficulties ae not immediately obvious. Howeve, it is known that on a elative basis the stong attenuato fo both epithemal and themal neutons is the liquid-filled boehole and a weak attenuato is the fomation. Thus, to incease sensitivity to the popeties of the fomation (o decease boehole sensitivity) the detecto should be positioned as fa fom the souce as possible, allowing those neutons which tavel towad the detecto within boehole to be attenuated elative to those taveling in the pope diection within the fomation. At least two neuton goups ae equied fo themal neuton computations, one fo epithemal neutons (enegies above 1eV), and one fo the themal neutons (enegies less than 1 ev). Theoetical Consideations Two goup neuton diffusion equations: D S 0 (1) D epith. epith. epithemal epith 0 () th. emal th. emal themal themal epithemal epithemal Whee: Φ - neuton flux, S is the neuton souce stength pe unit volume, D is the diffusion coefficient and Σ is emoval coss section. (*- explanation below). Solution of (1) and () fo the case of a point souce in an infinite homogeneous medium is: epith Q ) 4 D epith e Lepith ( (3) QL themal themal( ) 4D themal( Lepithemal Lthemal e ) Lepithemal e Lthemal (4) Whee: Q-is the point neuton souce stength (neuton/sec), is distance souce-detecto. Slowing down length Lepithemal and themal neuton diffusion length Lthemal ae defined by: L L epithemal D epithemal (5) D epithemal themal themal (6) themal

2 Poosity (%) L1 (cm) L (cm) Table 1- Epithemal and themal neuton paametes fo silicate sandstone satuated with salt wate containing ppm NaCl. The spatial behavio of Φthemal will be govened fo lage values of by L1 o L, depending upon which is lage. This is impotant because Table 1 shows that L1 is always lage than L fo salt wate satuated sandstone. Thus, fo sufficiently lage, the spatial distibution of themal neutons will be govened by an epithemal neuton paamete Lepithemal. Fo the low poosities, using exponential pat of equation 4 can be shown that pat which contains Lepithemal o slowing down length is always geate than pat which contains diffusion length of themal neutons. If is moe than 70cm, paamete which contains diffusion length is negligible. This effect is moe emphasized on highe poosities. (Figue 1 and ). Figue 1 Red colo epithemal pat of equation (4), blue colo themal pat of equation (4), black colo is diffeence between epithemal pat and themal pat of equation (4). Theoetical values fo silicate sandstone 30 pu, ppm of NaCl. Figue Red colo epithemal pat of equation (4), blue colo themal pat of equation (4), black colo is diffeence between epithemal pat and themal pat of equation (4). Theoetical values fo silicate sandstone 5 pu, ppm of NaCl.

3 This esult implies that fo souce detecto spacing of 70cm o geate the equation (4) can be ewite as a easonable appoximation: QL themal themal( ) 4D themal( Lepithemal Lthemal e ) Lepithemal (7) Although the spatial shape of this expession is govened by slowing down length, the magnitude of themal neuton flux is still scaled by themal neuton paametes. Clealy, the essentially all dependence of a themal neuton measuement upon themal neuton paametes can be eliminated simply by making measuements at two sufficiently distant points 1 and and taking thei atio; thus ( 1 ) ( ) 1 1 e 1 L epithemal This atio is not only independent of themal neuton diffusion length and coefficient, but Q (neuton souce stength) as well. Thee ae seveal easons fo wanting to make a themal neuton measuement which is sensitive only to slowing down length. Fist, slowing down length is pincipally detemined by the concentation of hydogen in a medium and hydogen concentation can be fequently be elated to ock poosity. (Notable exceptions ae zones containing low pessue gas o substantial amounts of bound hydogen.) It is also vey desiable fo the esponse to be independent of diffusion length because diffusion length vaies significantly with wate salinity and ock type. Of equal impotance, the measuable themal neuton density at any point in the boehole o fomation usually exceeds the measuable epithemal neuton density at that point by a consideable amount. (8) Poosity Epithemal paametes Themal paametes Fesh Wate Salt wate ppm L(cm) D(cm) L(cm) D(cm) L(cm) D(cm) * * * * Table Expeimental and computed neuton paametes, fo fast neutons mean enegy 4.46MeV. FIELD SIMULATION It has shown theoetically that themal neuton measuements made at lage souce detecto spacing espond almost exclusively to changes in L epithmal. Above conclusions was used on field example fo modeling PNN esponse. In august 007, PNN was logged in open hole, immediately afte OH logging. Open hole was logged with Westen Atlas 3600 seies equipment. Logged was CN435 and compensated density, CDL 7, with othe logs. Well was dilled with standad 8.5 bit. Dilling mud was in the boehole. Open hole log is shown on figue (3).

4 Based on poosity log, L and D fo epithemal and themal paametes wee calculated fo PNN based on the data shown in table. These data wee input in fomula 4. Paamete detecto souce distance is set as fo PNN, nea was 40cm, and fa was 54cm. Applying fomula (4) with these paametes, blue cuve was ceated as a atio between count ates on two detectos. Red cuve epesents logged atio cuve fom PNN log. Black cuve is logged atio cuve fom compensated neuton CN 435. This is shown in figue 4. It is clealy that blue and ed cuve does not fit with black one, which epesents neuton poosity cuves. Accoding to conclusions listed above, values fo nea=70cm and fa=9cm wee chosen. To fit as much as possible with CN 435 atio cuve, offset=- was added to simulated atio. This situation is pesented on figue 5. Figue 3 Open hole diagam. Blue is CN in the scale 60-0, ed is bulk density on the scale , and geen is GR in the scale Figue 4 Diffeent atio cuves. Black is atio cuve CN 435,, ed is logged PNN atio cuve, and blue is simulated atio cuve, using equation 4, with with nea=40cm and fa=54cm.

5 Figue 5 Diffeent atio cuves. Black is atio cuve CN 435, Figue 6 Black is logged poosity with CN 435 tool, and blue is simulated atio cuves, using equations 4, ed is simulated poosity cuves with longe spacing using with nea=70cm and fa=9cm same polynomial pesentation as fo CN 435, with tool constant 0.96, and blue one is PNN poosity cuve with the same polynomial pesentation. This appoach was tested on few othe wells whee was CN435 data available. Results wee same as on pevious example, so othe diagams wee not shown. Field, eal, data ae diffeent fom simulated data. Based on polynomial pesentation fom CN435, new polynomial was geneated to fit all data fom the well. Natual GR cuve was used to nomalize atio depend on the lithology. Both atios in combination wee used to ceate CN cuve, using idea that the tuth is somewhee between them.

6 Result is shown on figue 7. Faily good matching is achieved. Analyses of data showed that PNN obtained data ae inside of (+/-) 7% value logged with CN435, which can be accepted as a epeatability allowed fo CN435 tool. Following examples shows esults of the same pocedue applied on diffeent wells. Figue 7 Blue cuve is oiginal CN435 log, and ed cuve is obtained fom PNN log. Scale is +45pu ight and -15pu left.

7 Example A Well dilled 8.5 bit, poduced fom open hole, pefoated fom , wate salinity inside boehole ppm, two yeas befoe PNN logging, well was teating with acid fom 187m to 1358m. Bottom of the well 1367m. Casing shoe 187m. Results pesented on the figue 8. Example B In the well is tubing -7/8 to 70m, and depth in casing is up to 77m. Pefoated inteval 75m 735m. Fluid in the well was wate, and fomation wate salinity ppm. Well logged in shut in condition. Results pesented on the figue 9. Figue 8 Well A - Left - SSN blue, LSN ed. Right CN cuve blue, PNN poosity cuve ed.

8 Figue 9 Well B - Left - SSN blue, LSN ed. Right CN cuve blue, PNN poosity cuve ed.

9 CONCLUSION Two-goup neuton diffusion theoy was used in this papewok to study theoetical esponse of PNN tool fo poosity evaluation. In undistubed open hole conditions, theoetical esults obtained thu simple simulation wee vey simila to data ecoded on the field. Relation between atio cuve and poosity cuve fom open hole was established though same polynomial pesentation as fo the Westen Atlas CN tool seies 435. Changing distance between neuton geneato and two detectos was shown that elative numbe of themal neutons detected on two detectos (thei atio) can be used as a measue of single epithemal neuton paamete, the slowing down length of the fomation. Fo eal data keeping CN435 polynomial as a basic, it is possible to use only one polynomial pesentation fo all PNN tools with poosity calculations in the limits of acceptable. Well conditions must be taken in account in poosity evaluation.