Pulse Neutron Neutron (PNN) tool logging for porosity

Transcription

1 Pulse Neuton Neuton (PNN) tool logging fo poosity Some theoetical aspects Hotwell Handelsges.m.b.H Oedenbuge Stasse Klingenbach, AUSTRIA Tel.: +43 (0) Fax: +43 (0) office@hotwell.at best components simple design igoous testing no sacifice to pefomance Rules Fo The Diection Of The Mind-Descates, Rene RULE I The aim of ou studies must be the diection of ou mind so that it may fom solid and tue judents on whateve mattes aise. RULE IV Thee is need of a method fo investigating the tuth about things. RULE XX Having found the equations, we must pefom the opeations which we have left out, neve making use of multiplication when thee is scope fo division. RULE XXI If thee ae seveal such equations, all of them must be educed to a single one: namely to the one whose tems occupy fewest places in a seies of magnitudes in continued popotion; and its tems must be set out in the ode followed by the seies.

2 Intoduction Response of neuton logs: boehole size, type of fluid in the boehole o the position of the logging instument in the boehole, effect of mudcake in open-hole logging and cementing in casedhole applications. 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. 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. Theoetical Consideations Two goup neuton diffusion equations: D D epith. epith. epithemalepith S th. emal th. emal themalthemal epithemalepithemal 0 0 Φ - neuton flux, S is the neuton souce stength pe unit volume, D is the diffusion coefficient and Σ is emoval coss section. Solution of two equations fo the case of a point souce in an infinite homogeneous medium is: epith ( ) Q 4 D epith e Lepith QL themal themal ( ) 4Dthemal ( Lepithemal Lthemal e ) Lepithemal e Lthemal Q-is the point neuton souce stength (neuton/sec), is distance soucedetecto. Slowing down length Lepithemal and themal neuton diffusion length Lthemal ae defined by: L epithemal D epithemal epithemal L themal D themal themal

3 Theoetical Consideations cont. Red colo epithemal pat of equation, blue colo themal pat of equation, black colo is diffeence between epithemal pat and themal pat of equation. Theoetical values fo silicate sandstone 30 pu, ppm of NaCl. QL themal themal ( ) 4Dthemal ( Lepithemal Theoetical values fo silicate sandstone 5 pu, ppm of NaCl Lepithemal e Lthemal ) Theoetical Consideations cont. QL ( L themal themal ( ) 4Dthemal epithemal Lthemal e ) Lepithemal ( 1 ) e ( ) 1 L epithemal 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; 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. - 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.) -esponse should be independent of diffusion length because diffusion length vaies significantly with wate salinity and ock type. -the measuable themal neuton density at any point in the boehole o fomation usually exceeds the measuable epithemal neuton density Poosity Epithemal paametes Themal paametes Fesh Wate Salt wate ppm L(cm) D(cm) L(cm) D(cm) L(cm) D(cm) * * * * Expeimental and computed neuton paametes, fo fast neutons mean enegy 4.46MeV. 1 1

4 Field Expeiment Example A It has shown theoetically that themal neuton measuements made at lage souce detecto spacing espond almost exclusively to changes in Lepithmal. 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. Based on poosity log, L and D fo epithemal and themal paametes wee calculated fo PNN based on the data shown in table. 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. Red is bulk density on the scale , and blue is CN 435 in scale 60-0,geen GR in the scale Field Expeiment-Example A Accoding to conclusions listed above, values fo nea=70cm and fa=9cm wee chosen.

5 Field Expeiment Example A Same open hole well whee Total Ratio defined as atio SS/LS fom fist channel to 60, is used fo poosity calculation, modified with GR index. Field Expeiment Example B 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.

6 Field Expeiment Example B 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. Blue is PNN Ratio in scale 4-1., pink is PNN poosity cuve in scale 60-0, ed is CN cuve in scale 60-0 Field Expeiment Example C 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. Blue = PNN Ratio 10-0 Red = Nomalizied atio 5-0 Geen = GR nomalizied 1-0 Light Blue CNPOR = CasedHole CN 60-0 Violet = NPHI open hole CN 60-0

7 Field Expeiment Example C 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. CONCLUSIONS 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 Atals 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 polinomial 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.

8 Rules Fo The Diection Of The Mind-Descates, Rene RULE VIII If in the seies of subjects to be examined we come to a subject of which ou intellect cannot gain a good enough intuition, we must stop thee; and we must not examine the othe mattes that follow, but must efain fom futile toil.